Filed Pursuant to Rule 497​

 File No. 333-232387​

GOLUB CAPITAL BDC, INC.

Up to 33,451,902 Shares of Common Stock
Issuable Upon Exercise of Rights
to Subscribe for Such Shares

We are an externally managed, closed-end, non-diversified management investment company that has elected to be regulated as a business development company under the Investment Company Act of 1940, as amended, or the 1940 Act. Our investment objective is to generate current income and capital appreciation by investing primarily in one stop and other senior secured loans of U.S. middle-market companies. We can also selectively invest in second lien and subordinated loans of, and warrants and minority equity securities in, U.S. middle-market companies.

GC Advisors LLC serves as our investment adviser. Golub Capital LLC serves as our administrator. GC Advisors LLC and Golub Capital LLC are affiliated with Golub Capital (as defined herein), a leading lender to middle-market companies that has over $30.0 billion of capital under management as of December 31, 2019.

We issued transferable subscription rights to our stockholders of record as of 5:00 p.m., New York City time, on April 8, 2020, entitling the holders thereof to subscribe for up to an aggregate of 33,451,902 shares of our common stock. Record date stockholders received one right for each four outstanding shares of common stock owned on the record date. The rights entitle the holders to purchase one new share of common stock for every right held. In addition, record date stockholders who fully exercise their rights are entitled to subscribe, subject to the limitations described in this prospectus supplement and subject to allotment, for additional shares that remain unsubscribed as a result of any unexercised rights. Rights holders who exercise their rights will have no right to rescind their subscriptions after receipt of their completed subscription certificates together with payment for shares or a notice of guaranteed delivery by the subscription agent. Certain affiliates of Golub Capital LLC, an affiliate of GC Advisors LLC, have indicated that they intend to over-subscribe and to make a total investment of up to $125 million in shares of our common stock pursuant to exercise of the primary subscription and/or the over-subscription privilege.

As a result of the terms of this offering, stockholders who do not fully exercise their rights will own, upon completion of this offering, a smaller proportional interest in us than they owned prior to the offering. In addition, because the subscription price per share will likely be less than the net asset value per share, based on our current market price, the offering will likely result in an immediate dilution of net asset value per share for all of our stockholders. This offering will also cause dilution in the net investment income per each share of our common stock, which may affect the amount per share we are able to distribute subsequent to completion of the offering. Such dilution is not currently determinable because it is not known how many shares will be subscribed for or what the net asset value or market price of our common stock will be on the expiration date for the offer. If the subscription price per share is substantially less than the current net asset value per share, such dilution could be substantial. Any such dilution will disproportionately affect non-exercising stockholders. If the subscription price is less than our net asset value per share, then all stockholders will experience a decrease in the net asset value per share held by them, irrespective of whether they exercise all or any portion of their rights. See “Risk Factors — Your economic and voting interest in us, as well as your proportionate interest in our net asset value, could be diluted as a result of this rights offering” and “Dilution” in this prospectus supplement for more information.

After giving effect to the sale of shares of our common stock in this offering, as of December 31, 2019, assuming all rights are exercised at the estimated subscription price of  $11.45 per share and our receipt of the estimated net proceeds from that sale, our “as adjusted” net asset value would have been approximately $2.604 billion, or approximately $15.57 per share, representing immediate net asset value dilution of approximately $1.09 per share to our existing stockholders.

Our common stock is traded on the Nasdaq Global Select Market under the symbol “GBDC”. The last reported closing price for our common stock on April 9, 2020 was $12.39 per share. The net asset value of our common stock as of December 31, 2019 (the last date prior to the date of this prospectus supplement on which we determined net asset value) was $16.66 per share. The subscription rights are transferable and are traded on the Nasdaq Global Select Market under the symbol “GBDCR”. See “The Offering” for a complete discussion of the terms of this offering.

The subscription price per share will be 92.5% of the volume-weighted average of the sales prices of our shares of common stock on the Nasdaq Global Select Market for the five consecutive trading days ending on the expiration date of the offering. Because the subscription price will be determined on the expiration date, rights holders will generally not know the subscription price at the time of exercise. The rights will expire if they are not exercised by 5:00 p.m., New York City time, on May 6, 2020, the expiration date of the offering, unless extended as described in this prospectus supplement. We, in our sole discretion, can extend the period for exercising the subscription rights.

Shares of closed-end investment companies, including business development companies, frequently trade at a discount to their net asset value. If our shares continue to trade at a discount to our net asset value, it will likely increase the risk of loss for purchasers in this offering. Investing in our securities involves a high degree of risk. Before buying any securities, you should read the discussion of the material risks of investing in the rights and our common stock, including the risk of leverage, in “Risk Factors” beginning on page S-21 of this prospectus supplement and on page 8 of the accompanying prospectus or otherwise included in or incorporated by reference herein or the accompanying prospectus and in any free writing prospectuses we have authorized for use in connection with a specific offering, and under similar headings in the other documents that are incorporated by reference into this prospectus supplement and the accompanying prospectus.

This prospectus supplement and the accompanying prospectus contain important information you should know before investing in the rights. Please read this prospectus supplement and the accompanying prospectus, and the documents incorporated by reference herein and therein, before you invest and keep it for future reference. We file annual, quarterly and current reports, proxy statements and other information about us with the Securities and Exchange Commission, or the SEC. We maintain a website at http://www.golubcapitalbdc.com and make all of our annual, quarterly and current reports, proxy statements and other publicly filed information available on or through our website. Information on our website is not incorporated into or a part of this prospectus supplement or the accompanying prospectus or any free writing prospectus. You can also obtain such information, free of charge, and make shareholder inquiries by calling us collect at (212) 750-6060 or by contacting us at 200 Park Avenue, 25th Floor, New York, New York 10166, Attention: Investor Relations or investorrelations@golubcapital.com. The SEC also maintains a website at http://www.sec.gov that contains such information.

We generally invest in securities that have been rated below investment grade by independent rating agencies or that would be rated below investment grade if they were rated. These securities, which are also referred to as “junk,” have predominantly speculative characteristics with respect to the issuer’s capacity to pay interest and repay principal. In addition, many of our debt investments have floating interest rates that reset on a periodic basis and typically do not fully pay down principal prior to maturity, which could increase our risk of losing part or all of our investment.

(1)

Estimated on the basis of 92.5% of the volume-weighted average of the sales prices of our shares of common stock on the Nasdaq Global Select Market for the five consecutive trading days ending on March 30, 2020. See “The Offering — Subscription Price.”

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(2)

In connection with this offering, the dealer managers for this offering will receive a fee for marketing and soliciting services equal to 2% of the subscription price per share for each share issued other than any shares issued pursuant to exercise of the primary subscription and/or the over-subscription privilege other than affiliates of Golub Capital LLC, an affiliate of GC Advisors, and 1% of the subscription price per share for each share issued to affiliates of Golub Capital LLC. The estimated sales load assumes all shares are purchased other than by affiliates of Golub Capital LLC. See “The Offering — Distribution Arrangements.”

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(3)

We estimate that we will incur offering expenses of approximately $625,000 in connection with this offering. We estimate that net proceeds to us after expenses will be $374,705,340 assuming all of the rights are exercised at the estimated subscription price.

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(4)

Assumes all rights are exercised at the estimated subscription price.

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Neither the SEC nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

Lead Dealer Manager

Wells Fargo Securities

Co-Dealer Managers

The date of this prospectus supplement is April 13, 2020.

ABOUT THIS PROSPECTUS SUPPLEMENT

You should rely only on the information included or incorporated by reference in this prospectus supplement and the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us that relates to this offering. We have not, and the dealer managers have not, authorized any other person to provide you with different information. We are not, and the dealer managers are not, making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should not assume that the information appearing in this prospectus supplement, the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us that relates to this offering is accurate as of any date other than their respective dates, or that any information incorporated by reference herein or therein is accurate as of any date other than the date of the document incorporated by reference, regardless of the time of delivery of this prospectus supplement or the sale of our common stock offered hereby. It is possible that our business, financial condition, results of operations, cash flows and prospects have changed since that date. We will update these documents to reflect material changes only as required by law. We are offering to sell, and seeking offers to buy, securities only in jurisdictions where offers are permitted.

This document is in two parts. The first part is this prospectus supplement, which describes the terms of this offering and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference into this prospectus supplement and the accompanying prospectus. The second part is the accompanying prospectus, which gives more general information and disclosure. To the extent the information contained in this prospectus supplement differs from the information contained in the accompanying prospectus or any document filed prior to the date of this prospectus supplement and incorporated herein by reference, the information in this prospectus supplement will control. You should read this prospectus supplement, the accompanying prospectus and any documents incorporated by reference herein or therein and any free writing prospectus prepared by or on behalf of us that relates to this offering together with the additional information described under the headings “Risk Factors” included in this prospectus supplement and the accompanying prospectus and “Available Information” included in this prospectus supplement before you make an investment decision.

S-i

TABLE OF CONTENTS

PROSPECTUS SUPPLEMENT

PROSPECTUS

S-ii

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S-1

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S-2

S-3

S-4

S-5

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S-6

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Estimated to have totaled between:

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Net Investment Income Per Share

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​

​

​

​

​

​

​

​

​

​

​

​

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Net investment income per share

​

​

​

$

0.22

​

​

​

​

$

0.26

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​

​

Amortization of purchase premium per share(1)

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​

​

$

0.10

​

​

​

​

$

0.08

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​

​

Adjusted net investment income per share(1)

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​

​

$

0.32

​

​

​

​

$

0.34

​

​

​

Net Realized/Unrealized Gain (Loss) Per Share

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​

​

​

​

​

​

​

​

​

​

​

​

​

Net realized/unrealized gain (loss) per share

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​

​

$

(2.12)

​

​

​

​

$

(1.91)

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​

​

Reversal of unrealized loss resulting from the amortization of the purchase price premium per share(1)

​

​

​

$

(0.10)

​

​

​

​

$

(0.08)

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​

​

Adjusted net realized/unrealized gain (loss) per share(1)

​

​

​

$

(2.22)

​

​

​

​

$

(1.99)

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​

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Earnings per Share

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​

​

​

​

​

​

​

​

​

​

​

​

​

Earnings per share

​

​

​

$

(1.90)

​

​

​

​

$

(1.65)

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​

​

Adjusted earnings per share(1)

​

​

​

$

(1.90)

​

​

​

​

$

(1.65)

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​

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Net Asset Value per Share

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​

​

$

14.43

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​

​

​

$

14.68

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​

​

​

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S-7

S-8

S-9

S-10

S-11

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S-12

S-13

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S-14

FEES AND EXPENSES

The following table is intended to assist you in understanding the costs and expenses that an investor in shares of our common stock will bear directly or indirectly. However, we caution you that some of the percentages indicated in the table below are estimates and could vary. Actual costs and expenses incurred by investors in shares of our common stock could be greater than the percentage estimates in the table below. The following table excludes one-time fees payable to third parties not affiliated with GC Advisors that were incurred in connection with each of the Debt Securitizations, but includes all of the applicable ongoing fees and expenses of the Debt Securitizations. Whenever this prospectus supplement contains a reference to fees or expenses paid by “us” or “Golub Capital BDC,” or that “we” will pay fees or expenses, our common stockholders will indirectly bear such fees or expenses.

(1)

In connection with this offering, the dealer managers for this offering will receive a fee for marketing and soliciting services equal to 2% of the subscription price per share for each share issued pursuant to exercise of the primary subscription and/or the over-subscription privilege other than to affiliates of Golub Capital LLC, an affiliate of GC Advisors, and 1% of the subscription price per share for each share issued to affiliates of Golub Capital LLC, provided that the dealer managers are permitted to waive certain of the amounts to which it is entitled. See “The Offering — Distribution Arrangements.” This table assumes all shares purchased upon the exercise of rights are purchased by non-affiliates of Golub Capital.

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(2)

Amount reflects estimated offering expenses of approximately $625,000, which assumes that the offer is fully subscribed. See “The Offering — Distribution Arrangements.”

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(3)

The expenses associated with the dividend reinvestment plan are included in “Other expenses.” See “Dividend Reinvestment Plan” in the accompanying prospectus.

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(4)

Our management fee is calculated at an annual rate equal to 1.375% and is based on the average adjusted gross assets (including assets purchased with borrowed funds and securitization-related assets, leverage, unrealized depreciation or appreciation on derivative instruments and cash collateral on deposit with custodian but adjusted to exclude cash and cash equivalents so that investors do not pay the base management fee on such assets) at the end of the two most recently completed calendar quarters and is payable quarterly in arrears. See “Item 1. Business — Management Agreements —  Investment Advisory Agreement — Management Fee” included in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC. The management fee referenced in the table above is annualized and based on actual amounts incurred during the three months ended December 31, 2019 by GC Advisors in its capacity as investment adviser to us and collateral manager to each of the 2014 Issuer, the 2018 Issuer, and the GCIC 2018 Issuer, annualized for a full year.

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S-15

GC Advisors, as collateral manager for the 2018 Issuer, under a collateral management agreement, or the 2018 Collateral Management Agreement, is entitled to receive an annual fee in an amount equal to 0.25% of the principal balance of the portfolio loans held by the 2018 Issuer at the beginning of the collection period relating to each payment date, which is payable in arrears on each payment date. This fee, which is less than the management fee payable under the Investment Advisory Agreement, is paid directly by the 2018 Issuer to GC Advisors and offset against such management fee. Accordingly, the 1.375% base management fee paid by us to GC Advisors under the Investment Advisory Agreement on all of our assets, including those indirectly held through the 2018 Issuer, is reduced, on a dollar-for-dollar basis, by an amount equal to such 0.25% fee paid to GC Advisors by the 2018 Issuer. Under the 2018 Collateral Management Agreement, the term “collection period” refers to the period commencing on the third business day prior to the preceding payment period and ending on (but excluding) the third business day prior to such payment date. This fee can be waived by the collateral manager. The 2018 Collateral Management Agreement does not include any incentive fee payable to GC Advisors.

GC Advisors, as collateral manager for the 2014 Issuer, under a collateral management agreement, or the 2014 Collateral Management Agreement, is entitled to receive an annual fee in an amount equal to 0.25% of the principal balance of the portfolio loans held by the 2014 Issuer at the beginning of the collection period relating to each payment date, which is payable in arrears on each payment date. This fee, which is less than the management fee payable under the Investment Advisory Agreement, is paid directly by the 2014 Issuer to GC Advisors and offset against such management fee. Accordingly, the 1.375% base management fee paid by us to GC Advisors under the Investment Advisory Agreement on all of our assets, including those indirectly held through the 2014 Issuer, is reduced, on a dollar-for-dollar basis, by an amount equal to such 0.25% fee paid to GC Advisors by the 2014 Issuer. Under the 2014 Collateral Management Agreement, the term “collection period” refers to a quarterly period running from the day after the end of the prior collection period to the tenth business day prior to the payment date. This fee can be waived by the collateral manager. The 2014 Collateral Management Agreement does not include any incentive fee payable to GC Advisors.

GC Advisors, as collateral manager for the GCIC 2018 Issuer, under a collateral management agreement, or the GCIC 2018 Collateral Management Agreement, is entitled to receive an annual fee in an amount equal to 0.35% of the principal balance of the portfolio loans held by the GCIC 2018 Issuer at the beginning of the collection period relating to each payment date, which is payable in arrears on each payment date. This fee, which is less than the management fee payable under the Investment Advisory Agreement, is paid directly by the GCIC 2018 Issuer to GC Advisors and offset against such management fee. Accordingly, the 1.375% base management fee paid by us to GC Advisors under the Investment Advisory Agreement on all of our assets, including those indirectly held through the GCIC 2018 Issuer, is reduced, on a dollar-for-dollar basis, by an amount equal to such 0.35% fee paid to GC Advisors by the GCIC 2018 Issuer. Under the GCIC 2018 Collateral Management Agreement, the term “collection period” refers to a quarterly period running from the day after the end of the prior collection period to the tenth business day prior to the payment date. This fee can be waived by the collateral manager. The GCIC 2018 Collateral Management Agreement does not include any incentive fee payable to GC Advisors.

For purposes of this table, the SEC requires that the “Management fees” percentage be calculated as a percentage of net assets attributable to common stock, rather than total assets, including assets that have been funded with borrowed monies, because common stockholders bear all of this cost. If the base management fee portion of the “Management fees” percentage were calculated instead as a percentage of our total assets, our base management fee portion of the “Management fees” percentage would be approximately 1.30% of total assets. The base management fee in the table above is based on net assets of  $2.604 billion and leverage of  $2.053 billion, which reflects our net assets and leverage pro forma as of December 31, 2019 after giving effect to this offering and to the assumption of the SLF Credit Facility and the GCIC SLF Credit Facility.

(5)

The incentive fee referenced in the table above is based on actual amounts of the income component of the incentive fee incurred during the three months ended December 31, 2019, annualized for a full year, and the capital gains component payable under the Investment Advisory Agreement as of

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S-16

December 31, 2019. We have structured the calculation of the incentive fee to include a fee limitation such that no incentive fee will be paid to GC Advisors for any quarter if, after such payment, the cumulative incentive fees paid to GC Advisors, calculated on a per share basis since the effective date of our election to become a business development company would be greater than 20.0% of our Cumulative Pre-incentive Fee Net Income Per Share (as defined below).

The incentive fee in the Investment Advisory Agreement (the “Incentive Fee”) is comprised of two parts: the “Income Incentive Fee” component and the “Capital Gain Incentive Fee” component. The calculation of the Incentive Fees (the “Income and Capital Gains Incentive Fee Calculation”) has two parts.

“Pre-Incentive Fee Net Investment Income” under the Investment Advisory Agreement means for each quarter (or portion thereof) interest income, dividend income and any other income (including any other fees such as commitment, origination, structuring, diligence and consulting fees or other fees that we receive from portfolio companies, but excluding fees for providing managerial assistance) accrued during the period, minus operating expenses for the calendar quarter (including the base management fee, taxes, any expenses payable under the Investment Advisory Agreement and the administration agreement, or the Administration Agreement, with the Administrator, any expenses of securitizations and any interest expense and dividends paid on any outstanding preferred stock, but excluding the applicable incentive fees). Pre-Incentive Fee Net Investment Income includes, in the case of investments with a deferred interest feature such as market discount, debt instruments with payment-in-kind, or PIK, interest, preferred stock with PIK dividends and zero coupon securities, accrued income that we have not yet received in cash. Under the Investment Advisory Agreement, GC Advisors does not return amounts paid to it on accrued income that we have not yet received in cash if such income is not ultimately received by us in cash. If we do not ultimately receive income, a loss would be recognized, reducing future fees. Pre-Incentive Fee Net Investment Income does not include any realized capital gains, realized capital losses or unrealized capital appreciation or depreciation or any amortization or accretion of any purchase premium or purchase discount to interest income resulting solely from the purchase accounting for any premium or discount paid for the acquisition of assets in a merger, such as the premium to net asset value paid for the shares of GCIC common stock in the Merger.

Pre-Incentive Fee Net Investment Income, expressed as a rate of return on the value of our net assets (defined as total assets less indebtedness and before taking into account any incentive fees payable during the period) at the end of the immediately preceding calendar quarter, is compared to a fixed “hurdle rate” of 2.0% quarterly. If market interest rates rise, we could have the ability to invest funds in debt instruments that provide for a higher return, which would increase our Pre-Incentive Fee Net Investment Income and make it easier for GC Advisors to surpass the fixed hurdle rate and receive an incentive fee based on such net investment income. Pre-Incentive Fee Net Investment Income used to calculate this part of the incentive fee is also included in the amount of our total assets (excluding cash and cash equivalents but including assets purchased with borrowed funds and securitization-related assets and cash collateral on deposit with custodian) used to calculate the 1.375% base management fee, which fee is payable on all of our assets managed by GC Advisors.

We calculate the income component of the Income and Capital Gain Incentive Fee Calculation with respect to our Pre-Incentive Fee Net Investment Income quarterly, in arrears, as follows:

•

zero in any calendar quarter in which the Pre-Incentive Fee Net Investment Income does not exceed the hurdle rate;

​

•

100.0% of Pre-Incentive Fee Net Investment Income with respect to that portion of such Pre-Incentive Fee Net Investment Income, if any, that exceeds the hurdle rate but is less than 2.5% in any calendar quarter. We refer to this portion of our Pre-Incentive Fee Net Investment Income (which exceeds the hurdle rate but is less than 2.5%) as the “catch-up” provision. The catch-up is meant to provide GC Advisors with 20.0% of our Pre-Incentive Fee Net Investment Income as if a hurdle rate did not apply if this net investment income exceeds 2.5% in any calendar quarter; and

​

•

20.0% of the amount of our Pre-Incentive Fee Net Investment Income, if any, that exceeds 2.5% in any calendar quarter.

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S-17

The sum of these calculations yields the “Income Incentive Fee.” This amount is appropriately adjusted for any share issuances or repurchases during the quarter.

The Income Incentive Fee is calculated quarterly in arrears based on our Pre-Incentive Fee Net Investment Income for the immediately preceding calendar quarter, calculated as described above. Pre-Incentive Fee Net Investment Income does not include any realized capital gains, realized capital losses or unrealized capital appreciation or depreciation. Because of the structure of the income component, it is possible that an incentive fee could be calculated under this formula with respect to a period in which GBDC has incurred a loss. For example, if GBDC receives Pre-Incentive Fee Net Investment Income in excess of the hurdle rate (as defined below) for a calendar quarter, the income component will result in a positive value and an incentive fee will be paid subject to the Current GBDC Incentive Fee Cap.

The second part of the Income and Capital Gains Incentive Fee Calculation, the Capital Gain Incentive Fee, equals (a) 20.0% of our Capital Gain Incentive Fee Base, if any, calculated in arrears as of the end of each calendar year (or upon termination of the Investment Advisory Agreement, as of the termination date), commencing with the calendar year ending December 31, 2010 less (b) the aggregate amount of any previously paid Capital Gain Incentive Fees. The “Capital Gain Incentive Fee Base” under the Investment Advisory Agreement equals (1) the sum of  (i) our realized capital gains, if any, on a cumulative positive basis from April 13, 2010 through the end of each calendar year, (ii) all realized capital losses on a cumulative basis and (iii) all unrealized capital depreciation on a cumulative basis less (2) all unamortized deferred financing costs, if and to the extent such costs exceed all unrealized capital appreciation on a cumulative basis.

•

The cumulative aggregate realized capital losses are calculated as the sum of the amounts by which (a) the net sales price of each investment in GBDC’s portfolio when sold is less than (b) the accreted or amortized cost basis of such investment.

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•

The cumulative aggregate realized capital gains are calculated as the sum of the differences, if positive, between (a) the net sales price of each investment in GBDC’s portfolio when sold and (b) the accreted or amortized cost basis of such investment.

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The Investment Advisory Agreement includes a fee limitation such that no incentive fee will be paid to GC Advisors for any quarter if, after such payment, cumulative incentive fees paid to GC Advisors, calculated on a per share basis as described below, since the effective date of our election to become a business development company would be greater than 20.0% of GBDC’s Cumulative Pre-Incentive Fee Net Income Per Share (as defined below). Each quarterly incentive fee payable under the Income and Capital Gains Incentive Fee Calculation is subject to a cap, or the Incentive Fee Cap. The Incentive Fee Cap in any quarter is equal to the difference between (a) 20.0% of Cumulative Pre-Incentive Fee Net Income Per Share and (b) Cumulative Incentive Fees Paid Per Share (as defined below). To the extent the Incentive Fee Cap is zero or a negative value in any quarter, no incentive fee would be payable in that quarter. “Cumulative Pre-Incentive Fee Net Income Per Share” under the Investment Advisory Agreement is equal to the sum of Pre-Incentive Fee Net Income Per Share (as defined below) for each quarter since April 13, 2010. “Pre-Incentive Fee Net Income Per Share” for any quarter is equal to (a) the sum of  (i) Pre-Incentive Fee Net Investment Income and (ii) Adjusted Capital Returns (as defined below) for the quarter divided by (b) the weighted average number of shares of our common stock outstanding during such quarter. “Adjusted Capital Returns” for any quarter shall be the sum of the realized aggregate capital gains, realized aggregate capital losses, aggregate unrealized capital depreciation and aggregate unrealized capital appreciation for such quarter; provided that the calculation of realized aggregate capital gains, realized aggregate capital losses, aggregate unrealized capital depreciation and aggregate unrealized capital appreciation shall not include any realized capital gains, realized capital losses or unrealized capital appreciation or depreciation resulting solely from the purchase accounting for any premium or discount paid for the acquisition of assets in a merger. “Cumulative Incentive Fees Paid Per Share” is equal to the sum of Incentive Fees Paid Per Share for each quarter (or portion thereof) since April 13, 2010. “Incentive Fees Paid Per Share” for any quarter is equal to the incentive fees accrued and/or payable by GBDC for such period divided by the weighted average number of shares of common stock of GBDC outstanding during such period. For a more

S-18

detailed discussion of the calculation of the incentive fee, see “Item 1. Business — Management Agreements — Income and Capital Gains Incentive Fee Calculation” included in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC.

(6)

Interest payments on borrowed funds is based on our cost of funds on our outstanding indebtedness for the three months ended December 31, 2019, which consisted of  $890.7 million of indebtedness outstanding under Revolving Credit Facilities, $305.0 million of debentures issued through our small business investment company (“SBIC”) subsidiaries, $1.074 billion in notes issued through the Debt Securitizations and $65.8 million of other short term borrowings. For the three months ended December 31, 2019, the annualized cost of funds for our total debt outstanding, which includes all interest and amortization of debt issuance costs on the Debt Securitizations and Revolving Credit Facilities, was 3.88%. Debt issuance costs represent fees and other direct incremental costs incurred in connection with our Debt Securitizations and Revolving Credit Facilities. These fees include a structuring and placement fee paid to Wells Fargo Securities, LLC for its services in connection with the initial structuring of the 2014 Debt Securitization of  $1.81 million, certain fees in an aggregate amount of  $0.35 million paid to J.P. Morgan Securities Inc. in connection with subsequent amendments and legal fees, accounting fees, rating agency fees and all other costs associated with the 2014 Debt Securitization. These fees also include a structuring and placement fee paid to Morgan Stanley & Co. LLC for its services in connection with the initial structuring of the 2018 Debt Securitization of  $1.20 million and legal fees, accounting fees, rating agency fees and all other costs associated with the 2018 Debt Securitization.

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(7)

Includes our overhead expenses, including payments under the Administration Agreement based on our allocable portion of overhead and other expenses incurred by the Administrator, and any acquired fund fees and expenses that are not required to be disclosed separately. See “Item 1. Business — Management Agreements — Administration Agreement” included in our most Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC. “Other expenses” are estimated based on the annualized amounts incurred for the three months ended December 31, 2019.

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(8)

Prior to January 1, 2020, our stockholders indirectly bore the expenses of our investment in SLF and GCIC SLF. No management fees were charged by the Administrator in connection with the administrative services it provided to SLF and GCIC SLF. However, SLF and GCIC SLF reimbursed the Administrator for its costs related to providing accounting, bookkeeping, treasury, loan operations, reporting and administrative services for SLF and GCIC SLF. On January 1, 2020, we acquired the interests in SLF and GCIC SLF previously held by RGA Reinsurance Company, or RGA, and Aurora National Life Assurance Company, a wholly-owned subsidiary of RGA, or Aurora, respectively. As a result of the transactions, on January 1, 2020, SLF and GCIC SLF became wholly-owned subsidiaries of us. As wholly-owned and controlled subsidiaries, the asset and liabilities of SLF and GCIC SLF will be consolidated into our financial statements and notes thereto for periods ending on or after January 1, 2020.

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(9)

“Total annual expenses” as a percentage of consolidated net assets attributable to common stock are higher than the total annual expenses percentage would be for a company that is not leveraged. We borrow money to leverage our net assets and increase our total assets. The SEC requires that the “Total annual expenses” percentage be calculated as a percentage of net assets (defined as total assets less indebtedness and before taking into account any incentive fees payable during the period), rather than the total assets, including assets that have been funded with borrowed monies. The reason for presenting expenses as a percentage of net assets attributable to common stockholders is that our common stockholders bear all of our fees and expenses.

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Example

The following example demonstrates the projected dollar amount of total cumulative expenses that would be incurred over various periods with respect to a hypothetical investment in our common stock. This example and the expenses in the table above should not be considered a representation of our future expenses, and actual expenses (including the cost of debt, if any, and other expenses) may be greater or less than those shown.

The foregoing table is to assist you in understanding the various costs and expenses that an investor in our common stock will bear directly or indirectly. While the example assumes, as required by the SEC, a 5% annual return, our performance will vary and may result in a return greater or less than 5%. The incentive fee under the Investment Advisory Agreement, which, assuming a 5% annual return, would either not be payable or have an immaterial impact on the expense amounts shown above, is not included in the example. Under our Investment Advisory Agreement, no incentive fee would be payable if we have a 5% annual return. If we achieve sufficient returns on our investments, including through the realization of capital gains, to trigger an incentive fee of a material amount, our expenses, and returns to our investors, would be higher. The example assumes that all dividends and other distributions are reinvested at net asset value. Under certain circumstances, reinvestment of dividends and other distributions under our dividend reinvestment plan may occur at a price per share that differs from net asset value. See “Dividend Reinvestment Plan” in the accompanying prospectus for more information.

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RISK FACTORS

Before you invest in our securities, you should be aware of various risks, including those described below and those set forth in the accompanying prospectus or otherwise incorporated by reference herein or the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us that relates to this offering. You should carefully consider these risk factors, together with all of the other information included or incorporated by reference in this prospectus supplement and the accompanying prospectus, before you decide whether to make an investment in our securities. The risks set out below are not the only risks we face. If any of the following events occur, our business, financial condition and results of operations could be materially and adversely affected. In such case, our net asset value and the trading price of our securities could decline, and you could lose all or part of your investment.

Events outside of our control, including public health crises, could negatively affect our portfolio companies and our results of our operations.

Periods of market volatility have occurred and could continue to occur in response to pandemics or other events outside of our control. These types of events have adversely affected and could continue to adversely affect operating results for us and for our portfolio companies. For example, in December 2019, a novel strain of coronavirus (also known as “COVID-19”) surfaced in China and has since spread to other countries, including the United States, which has resulted in restrictions on travel and the temporary closure of many corporate offices, retail stores, and manufacturing facilities and factories in the affected jurisdictions, including the United States. In addition to these developments having adverse consequences for us and our portfolio companies and, the operations of GC Advisors (including those relating to us) have been, and could continue to be, adversely impacted, including through quarantine measures and travel restrictions imposed on its personnel or service providers based or temporarily located in affected countries, or any related health issues of such personnel or service providers. As the potential impact of the coronavirus is difficult to predict, the extent to which the coronavirus could negatively affect our and our portfolio companies’ operating results or the duration of any potential business or supply-chain disruption, is uncertain. Any potential impact to our results of operations will depend to a large extent on future developments and new information that could emerge regarding the duration and severity of the coronavirus and the actions taken by authorities and other entities to contain the coronavirus or treat its impact, all of which are beyond our control. These potential impacts, while uncertain, could adversely affect our and our portfolio companies’ operating results.

We are currently operating in a period of capital markets disruption and economic uncertainty.

The U.S. capital markets have experienced extreme volatility and disruption following the global outbreak of COVID-19 that began in December 2019. Some economists and major investment banks have expressed concern that the continued spread of the virus globally could lead to a world-wide economic downturn. Disruptions in the capital markets have increased the spread between the yields realized on risk-free and higher risk securities, resulting in illiquidity in parts of the capital markets. These and future market disruptions and/or illiquidity would be expected to have an adverse effect on our business, financial condition, results of operations and cash flows. Unfavorable economic conditions also would be expected to increase our funding costs, limit our access to the capital markets or result in a decision by lenders not to extend credit to us. These events have limited and could continue to limit our investment originations, limit our ability to grow and have a material negative impact on our operating results and the fair values of our debt and equity investments.

Your economic and voting interest in us, as well as your proportionate interest in our net asset value, could be diluted as a result of this rights offering.

Stockholders who do not fully exercise their rights should expect that they will, at the completion of the offer, own a smaller proportional interest in us, including with respect to voting rights, than would otherwise be the case if they fully exercised their rights. We cannot state precisely the amount of any such dilution in share ownership because we do not know at this time what proportion of the shares of common stock will be purchased as a result of the offer.

In addition, if the subscription price is less than our net asset value per share, then our stockholders would experience an immediate dilution of the aggregate net asset value of their shares of common stock as

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a result of the offer. The amount of any decrease in net asset value is not predictable because it is not known at this time what the subscription price and net asset value per share will be on the expiration date of the rights offering or what proportion of the shares of common stock will be purchased as a result of the offer. Such dilution could be substantial.

This offering could also cause dilution in the net investment income per share of our common stock, which may affect the amount we are able to distribute subsequent to completion of the offering. In addition, our reported earnings per share will be retroactively adjusted to reflect the dilutive effects of this offering. See “Dilution.”

We have the ability to terminate this rights offering at any time prior to delivery of the shares of our common stock offered hereby, and neither we nor the subscription agent will have any obligation to you except to return your subscription payments, without interest.

We have the ability to terminate the rights offering at any time prior to the delivery of the shares of our common stock offered hereby. If the rights offering is terminated, all rights will expire without value and the subscription agent will return as soon as practicable all exercise payments, without interest. No amounts paid to acquire rights on the Nasdaq Global Select Market or otherwise will be returned.

There can be no assurance that a market for the rights will develop.

There can be no assurance that a market for the rights will develop or, if such a market develops, what the price of the rights will be. Changes in market conditions could result in the shares of common stock purchasable upon exercise of the rights being less attractive to investors at the expiration date. This could reduce or eliminate the value of the rights. Stockholders who receive or acquire rights could find that there is no market to sell rights that they do not wish to exercise.

Sales of substantial amounts of our common stock in the public market could have an adverse effect on the market price of our common stock.

Upon completion of this offering, we will have 167,257,666 shares of common stock outstanding if the offering is fully subscribed. Following this offering, sales of substantial amounts of our common stock, or the availability of such shares for sale, could adversely affect the prevailing market prices for our common stock. If this occurs and continues, it could impair our ability to raise additional capital through the sale of equity securities should we desire to do so.

If the current period of capital market disruption and instability continues for an extended period of time, there is a risk that investors in our equity securities may not receive distributions consistent with historical levels or at all or that our distributions may not grow over time and a portion of our distributions may be a return of capital.

We intend to make distributions on a quarterly basis to our stockholders out of assets legally available for distribution. We cannot assure you that we will achieve investment results that will allow us to make a specified level of cash distributions or year-to-year increases in cash distributions. Our ability to pay distributions might be adversely affected by the impact of one or more of the risk factors described in this prospectus supplement or incorporated herein by reference, including the COVID-19 pandemic described above. For example, if the temporary closure of many corporate offices, retail stores, and manufacturing facilities and factories in the jurisdictions, including the United States, affected by the COVID-19 pandemic were to continue for an extended period of time it could result in reduced cash flows to us from our existing portfolio companies, which could reduce cash available for distribution to our stockholders. If we are unable to satisfy the asset coverage test applicable to us under the 1940 Act as a business development company or if we violate certain covenants under our existing or future credit facilities or other leverage, we may be limited in our ability to make distributions. If we declare a distribution and if more stockholders opt to receive cash distributions rather than participate in our dividend reinvestment plan, we may be forced to sell some of our investments in order to make cash distribution payments. To the extent we make distributions to stockholders that include a return of capital, such portion of the distribution essentially constitutes a return of the stockholder’s investment. Although such return of capital may not be taxable, such distributions would generally decrease a stockholder’s basis in our common stock and may therefore

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increase such stockholder’s tax liability for capital gains upon the future sale of such stock. A return of capital distribution may cause a stockholder to recognize a capital gain from the sale of our common stock even if the stockholder sells its shares for less than the original purchase price.

There are material limitations with making preliminary estimates of our financial results for the three months ended March 31, 2020 prior to the completion of our and our auditors’ financial review procedures for such period.

The preliminary financial estimates contained in “Prospectus Supplement Summary — Recent Developments” are not a comprehensive statement of our financial results for the three months ended March 31, 2020 and have not been audited, reviewed, compiled, examined or subject to any procedures by our independent registered public accounting firm or any other independent accountants. Our consolidated financial statements for the three months ended March 31, 2020 will not be available until after this offering is completed and, consequently, will not be available to you prior to investing in this offering. Our actual financial results for the three months ended March 31, 2020 could differ materially from the preliminary financial estimates we have provided as a result of the completion of our financial closing procedures and related internal controls over financial reporting, final adjustments, execution of our disclosures and procedures and other developments arising between now and the time that our financial results for the three months ended March 31, 2020 are finalized. The preliminary financial data included herein has been prepared by, and is the responsibility of, management. Our independent registered public accounting firm has not audited, reviewed, compiled, examined or performed any procedures with respect to such preliminary estimates, and, accordingly, does not express an opinion or any other form of assurance with respect thereto. We intend to announce final results of operations as of and for the three and six months ended March 31, 2020 on or around May 11, 2020. These final results of operations will include financial information not included in this prospectus supplement, including, for example, a schedule of investments, industry and geographic information on our portfolio and internal performance ratings.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

Some of the statements in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference into this prospectus supplement and the accompanying prospectus and any free writing prospectus prepared by or on behalf of us that relates to this offering of our common stock constitute forward-looking statements, which relate to future events or our future performance or financial condition. The forward-looking statements contained in this prospectus supplement, the accompanying prospectus, including the documents incorporated by and any related free writing prospectus, involve risks and uncertainties, including statements as to:

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our future operating results;

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our business prospects and the prospects of our portfolio companies, including our and their ability to achieve our respective objectives as a result of the current COVID-19 pandemic;

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preliminary estimates of our financial condition and results of operations as of and for the three months ended March 31, 2020;

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the effect of investments that we expect to make and the competition for those investments;

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our contractual arrangements and relationships with third parties;

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•

actual and potential conflicts of interest with GC Advisors and other affiliates of Golub Capital;

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•

the dependence of our future success on the general economy and its effect on the industries in which we invest;

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the ability of our portfolio companies to achieve their objectives;

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the use of borrowed money to finance a portion of our investments;

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the adequacy of our financing sources and working capital;

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the timing of cash flows, if any, from the operations of our portfolio companies;

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•

general economic and political trends and other external factors, including the current COVID-19 pandemic;

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the ability of GC Advisors to locate suitable investments for us and to monitor and administer our investments;

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the ability of GC Advisors or its affiliates to attract and retain highly talented professionals;

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our ability to qualify and maintain our qualification as a RIC and as a business development company;

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general price and volume fluctuations in the stock markets;

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the impact on our business of the Dodd-Frank Wall Street Reform and Consumer Protection Act and the rules and regulations issued thereunder and any actions toward repeal thereof; and

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the effect of changes to tax legislation and our tax position.

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Such forward-looking statements may include statements preceded by, followed by or that otherwise include the words “may,” “might,” “will,” “intend,” “should,” “could,” “can,” “would,” “expect,” “believe,” “estimate,” “anticipate,” “predict,” “potential,” “plan” or similar words. The forward-looking statements contained in this prospectus supplement and the accompanying prospectus involve risks and uncertainties. Our actual results could differ materially from those implied or expressed in the forward-looking statements for any reason, including the factors set forth as “Risk Factors” in the accompanying prospectus and elsewhere in this prospectus supplement and the accompanying prospectus.

We have based the forward-looking statements included in this prospectus supplement or the accompanying prospectus, including any documents incorporated by reference, on information available to us on the date of this prospectus supplement. Actual results could differ materially from those anticipated in our forward-looking statements and future results could differ materially from historical performance. You are advised to consult any additional disclosures that we make directly to you or through reports that we have filed or in the future file with the SEC, including annual reports on Form 10-K, registration statements on Form N-2, quarterly reports on Form 10-Q and current reports on Form 8-K. This prospectus supplement and the accompanying prospectus, including any documents incorporated by reference, contain statistics and other data that have been obtained from or compiled from information made available by third-party service providers. We have not independently verified such statistics or data.

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USE OF PROCEEDS

We estimate that net proceeds we will receive from this offering will be approximately $374.7 million assuming all of the rights are exercised at the estimated subscription price of  $11.45 and after deducting estimated offering expenses of approximately $625,000 payable by us and payments to the dealer managers of 2% of the estimated subscription price per share for each share issued pursuant to exercise of the primary subscription and/or the over-subscription privilege other than to affiliates of Golub Capital LLC, assuming, solely for purposes of this calculation, that no affiliates of Golub Capital LLC subscribe for shares.

We intend to use the net proceeds from the sale of shares of our common stock primarily (i) to initially repay outstanding indebtedness and otherwise support our financing subsidiaries, (ii) to make additional investments to support our existing portfolio companies, (iii) to position our balance sheet to be in position to fund new investments in accordance with our investment objectives and strategies and (iv) for general corporate purposes. We expect that our new investments will consist primarily of one stop and other senior secured loans. We will also pay operating expenses, including management and administrative fees, and possibly other expenses such as due diligence expenses relating to potential new investments, from the net proceeds of this offering.

The indebtedness we may repay with the net proceeds of this offering includes amounts outstanding under our Revolving Credit Facilities, the SLF Credit Facility and the GCIC SLF Credit Facility. For the three months ended December 31, 2019, the WF Credit Facility had an annualized average stated interest rate of 3.8% (i.e., one-month LIBOR plus 2.00%) on the outstanding balance of  $274.0 million; the DB Credit Facility had an annualized average stated interest rate of 4.1% (i.e., applicable base rate plus 1.90%) on the outstanding balance of  $231.5 million; and the MS Credit Facility II had an annualized average stated interest rate of 3.8% (i.e., one-month LIBOR plus 2.05%) on the outstanding balance of $385.2 million. For the three months ended December 31, 2019, the SLF Credit Facility had an annualized average stated interest rate of 3.8% (i.e., one-month LIBOR plus 2.05%) on the outstanding balance of $52.3 million; and the GCIC SLF Credit Facility had an annualized average stated interest rate of 3.8% (i.e., one-month LIBOR plus 2.05%) on the outstanding balance of  $44.4 million. In addition, a portion of the net proceeds could be used to increase the capitalization of SBIC VI, L.P. (“SBIC VI”), one of our subsidiaries that that is licensed to operate as a SBIC by the Small Business Administration (the “SBA”), following which we would expect SBIC VI to issue additional debentures guaranteed by the SBA and make investments in accordance with our investment strategy.

We anticipate that we will use substantially all of the net proceeds of this offering for the above purposes within approximately six months after the completion of this offering, depending on the availability of appropriate investment opportunities consistent with our investment objective and market conditions. We cannot assure you that we will achieve our targeted investment pace.

Until appropriate investment opportunities can be found, we may invest the net proceeds of this offering primarily in cash, cash equivalents, U.S. government securities and high-quality debt investments that mature in one year or less from the date of investment. These temporary investments typically have lower yields than our other investments and, accordingly, could result in lower distributions, if any, during such period. Our ability to achieve our investment objective could be limited to the extent that the net proceeds from this offering, pending full investment, are held in lower yielding interest-bearing deposits or other short-term instruments. See “Item 1. Business — Regulation — Temporary Investments” included in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC, for additional information about temporary investments we could choose to make while waiting to make longer-term investments in pursuit of our investment objective.

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CAPITALIZATION

The following table sets forth:

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our actual capitalization as of December 31, 2019; and

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our pro forma capitalization to give effect to (i) the acquisition of the interests in SLF and GCIC SLF previously held by RGA and Aurora, respectively, and the assumption of approximately $96.7 million of debt that was outstanding as of December 31, 2019 under the SLF Credit Facility and the GCIC SLF Credit Facility and (ii) the sale of 33,451,902 shares of our common stock in this offering, assuming all rights are exercised at an estimated subscription price of  $11.45 per share and our receipt of the estimated net proceeds from that sale assuming, for purposes of the pro forma capitalization, that no affiliates of Golub Capital LLC subscribe for shares.

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DILUTION

The net asset value, or NAV, dilution to investors in this offering will be represented by the difference between the subscription price and the pro forma NAV per share of our common stock after this offering. NAV per share is determined by dividing our NAV, which is our total tangible assets less total liabilities, by the number of outstanding shares of common stock.

As of December 31, 2019, our net assets were $2.2 billion, or approximately $16.66 per share. After giving effect to the sale of 33,451,902 shares of our common stock in this offering, assuming all rights are exercised at an estimated subscription price per share of 11.45, and our receipt of the estimated net proceeds from that sale, our pro forma NAV as of December 31, 2019, including the acquisition of the interests in SLF and GCIC SLF previously held by RGA and Aurora, respectively, and the assumption of the SLF Credit Facility and the GCIC SLF Credit Facility as of such date, would have been approximately $2.604 billion, or approximately $15.57 per share, representing an immediate dilution of approximately $1.09 per share to our existing stockholders. This offering will also cause dilution in the net investment income per each share of our common stock, which may affect the amount we are able to distribute per share subsequent to completion of the offering.

The following table illustrates the dilutive effects of this offering on a per share basis, assuming all rights are exercised at an estimated subscription price per share of  $11.45:

(1)

Basic and diluted, weighted average number of shares outstanding is 132,683,147.

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(2)

Assumes that on October 1, 2019, the beginning of the indicated period, (1) all rights were exercised at an estimated subscription price per share of  $11.45, (2) 33,451,902 shares of our common stock were issued upon exercise of such rights and (3) no affiliates of Golub Capital subscribed for shares.

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(3)

Assumes actual cash distributions divided by adjusted shares, including shares issued upon exercise of rights.

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(4)

Due to the purchase accounting for the GCIC acquisition, as a supplement to U.S. generally accepted accounting principles (“GAAP”) financial measures, we are providing additional non-GAAP measures. Adjusted net increase in net assets resulting from net investment income per common share excludes the amortization of the purchase premium resulting from the GCIC acquisition. We believe that excluding the financial impact of the purchase premium in the above non-GAAP financial measures is useful for investors as this is a non-cash expense/loss and is one method we use to measure our financial condition and results of operations.

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(5)

Excludes a $0.13 special dividend paid during the three months ended December 31, 2019 that is not recurring. Including the special dividend, total distributions paid by us during the three months ended December 31, 2019 were $0.46 per common share and the as adjusted distribution per common share is $0.36 per share.

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SALES OF COMMON STOCK BELOW NET ASSET VALUE

If the subscription price per share pursuant to this rights offering is less than our NAV per share, shares of our common stock will be sold below NAV. Our board of directors, including our independent directors, has (a) determined that the sale of shares of our common stock below NAV pursuant to the exercise of the rights issued in this offering is in our best interests and in the best interests of our stockholders (including those stockholders who do not exercise their rights in the offering) and (b) in consultation with the dealer managers, determined in good faith that as of a time immediately prior to the issuance of such common stock, the subscription price for the shares of common stock to be issued upon exercise of the rights will be not less than a price that closely approximates the market value of those shares of common stock, less any distributing commission or discount.

In making a determination that the sale of common stock below NAV per share pursuant to the exercise of the rights issued in this rights offering is in our and our stockholders’ best interests, our board of directors considered a variety of factors including:

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the effect that a sale of common stock below NAV per share would have on our stockholders, including the potential dilution to the NAV per share of our common stock our stockholders would experience as a result of the issuance, including dilution for those stockholders who do not fully exercise the Rights issued to them in the offering;

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the amount per share by which the subscription price per share and the net proceeds per share are less than our most recently determined NAV per share;

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the relationship of recent market prices of our common stock to NAV per share and the potential impact of the issuance on the market price per share of our common stock;

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whether the estimated subscription price would closely approximate the market value of shares of our common stock;

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the potential market impact of being able to raise capital during the current financial market difficulties;

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the anticipated rate of return on and quality, type and availability of investments; and

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the leverage available to us.

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Our board of directors also considered the fact that GC Advisors will benefit from this offering because GC Advisors will earn additional investment management fees on the proceeds of the sale of shares of common stock upon exercise of rights in the same manner as it would from the offering of any other of our securities or from the offering of common stock at premium to NAV per share.

The sale by us of our common stock at a discount to NAV per share upon the exercise of the rights issued in this offering poses potential risks for our existing stockholders whether or not they participate in this rights offering. Any sale of common stock at a price below NAV per share results in an immediate dilution to our existing common stockholders who do not fully exercise the rights issued to them in this rights offering to purchase their pro rata portion of the shares of common stock issued. See “Risk Factors — Your economic and voting interest in us, as well as your proportionate interest in our NAV, could be diluted as a result of this rights offering.”

The following two headings and accompanying tables explain and provide hypothetical examples on the impact of the rights offering if the subscription price is less than NAV per share on three different types of investors:

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existing stockholders who do not participate in the rights offering; and

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existing stockholders who subscribe for a relatively small amount of shares in the rights offering or a relatively large amount of shares in the rights offering.

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Impact on Existing Stockholders Who Do Not Participate in the Rights Offering

If shares of our common stock are sold below NAV per share pursuant to the exercise of the rights issued in this offering, our existing stockholders who do not exercise the rights issued to them or who do not buy additional shares in the secondary market at the same or lower price as we obtain for subscriptions in the rights offering (after expenses and commissions) face the greatest potential risks. These stockholders will experience an immediate dilution in the NAV of the shares of common stock they hold and their NAV per share. These stockholders will also experience a disproportionately greater decrease in their participation in our earnings and assets and their voting power than the increase we will experience in our assets, potential earning power and voting interests due to such rights offering. These stockholders could also experience a decline in the market price of their shares, which often reflects to some degree announced or potential increases and decreases in NAV per share. This decrease could be more pronounced as the size of the issuance pursuant to the rights offering and level of discounts increase. Further, if existing stockholders do not exercise their right to purchase any shares in order to maintain their percentage interest, regardless of whether such issuance is above or below the then current NAV, their voting power will be diluted.

The following chart illustrates the level of NAV dilution that would be experienced by a non-participating stockholder in three different hypothetical issuances of different sizes and levels of discount from NAV per share. It is not possible to predict the level of market price decline that could occur.

The examples assume that the issuer has 100 million shares outstanding, $2 billion in total assets and $500 million in total liabilities. The current NAV and NAV per share are thus $1.5 billion and $15.00. The chart illustrates the dilutive effect on Stockholder A of  (a) an issuance of 5 million shares of common stock (5% of the outstanding shares) purchased at a subscription price of  $14.25 per share before expenses and dealer manager fees (a 5% discount from NAV), (b) an issuance of 10 million shares of common stock (10% of the outstanding shares) purchased at a subscription price of  $13.50 per share before expenses and dealer manager fees (a 10% discount from NAV), (c) an issuance of 20 million shares of common stock (20% of the outstanding shares) purchased at a subscription price of  $12.00 per share before expenses and dealer manager fees (a 20% discount from NAV) and (d) an issuance of 25 million shares of common stock (25% of the outstanding shares) purchased at a subscription price of  $11.25 per share before expenses and dealer manager fees (a 25% discount from NAV). The example assumes a dealer manager fee of 2.0% is paid on all shares.

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Impact on Existing Stockholders Who Do Participate in the Rights Offering

If shares of our common stock are sold below NAV per share pursuant to the exercise of the rights issued in this offering, our existing stockholders who participate in the offering or who buy additional shares in the secondary market at the same or lower price as we obtain in the rights offering (after expenses and commissions) will experience the same types of NAV dilution as the non-participating stockholders, although at a lower level, to the extent they subscribe for less than the same percentage of the discounted offering as their interest in shares of our common stock immediately prior to the rights offering. The level of NAV dilution will decrease as the number of shares for which such stockholders subscribe increases. Existing stockholders who subscribe for more than such percentage will experience NAV dilution but will, in contrast to existing stockholders who subscribe for less than their proportionate share of common stock prior to the rights offering, experience accretion in NAV per share over their investment per share and will also experience a disproportionately greater increase in their participation in our earnings and assets and their voting power than our increase in assets, potential earning power and voting interests due to such offering. The level of accretion will increase as the excess number of shares for which such stockholders subscribe increases. Even a stockholder who over-subscribes will, however, be subject to the risk that we could make additional discounted offerings in which such stockholder does not participate, in which case such a stockholder will experience NAV dilution as described above in such subsequent offerings. These stockholders could also experience a decline in the market price of their shares, which often reflects to some degree announced or potential increases and decreases in NAV per share. This decrease could be more pronounced as the size of the issuance and level of discounts increase.

The following chart illustrates the level of dilution and accretion in the hypothetical 20% discount issuance from the prior chart (Example 3) for a stockholder that subscribes for shares equal to (a) 50% of its proportionate share of the issuance (i.e., 10,000 shares, which is 0.05% of an issuance of 20 million shares) rather than its 0.10% proportionate share and (b) 150% of such percentage (i.e., 30,000 shares, which is 0.15% of an issuance of 20 million shares rather than its 0.10% proportionate share).

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PRICE RANGE OF COMMON STOCK AND DISTRIBUTIONS

Our common stock is currently traded on the Nasdaq Global Select Market under the symbol “GBDC”. The following table sets forth: (i) the NAV per share of our common stock as of the applicable period end, (ii) the range of high and low closing sales prices of our common stock as reported on the Nasdaq Global Select Market during the applicable period, (iii) the closing high and low sales prices as a premium (discount) to NAV during the relevant period, and (iv) the dividends and distributions per share of our common stock declared during the applicable period.

(1)

NAV per share is determined as of the last day in the relevant quarter and therefore does not necessarily reflect the NAV per share on the date of the high and low closing sales prices. The NAVs shown are based on outstanding shares at the end of each period.

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(2)

Calculated as of the respective high or low closing sales price divided by the quarter-end NAV.

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(3)

On April 9, 2020, our board of directors declared a quarterly distribution of  $0.29 per share which is payable in cash on June 29, 2020 to holders of record of our common stock as of June 9, 2020.

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(4)

Includes a special distribution of  $0.13 per share.

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(5)

Includes a special distribution of  $0.12 per share.

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(6)

Includes a special distribution of  $0.08 per share.

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The last reported price for our common stock on April 9, 2020 was $12.39 per share. As of April 8, 2020, we had 642 stockholders of record.

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THE OFFERING

Purpose of the Offering

Our board of directors has determined that the offering would result in a net benefit to the stockholders and that it is in the best interest of us and our stockholders to raise additional capital primarily (i) to initially repay outstanding indebtedness and otherwise support our financing subsidiaries, (ii) to make additional investments to support our portfolio companies, (iii) to position our balance sheet to be in position to fund new investments in accordance with our investment objectives and strategies and (iv) for general corporate purposes. The current offering gives existing stockholders the right to purchase additional shares at a price that is expected to be below the then-current trading price without incurring any commission or other charges, while providing us access to such additional capital resources. All costs of this rights offering will be borne by our stockholders whether or not they exercise subscription rights. In connection with the approval of this rights offering, our board of directors considered the following factors:

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the subscription price relative to the market price and to our NAV per share, including the substantial likelihood that the subscription price will be below our NAV per share and the resulting effect that the offering could have on our NAV per share;

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the increased capital to be available upon completion of the rights offering for us to make additional investments consistent with our investment objective;

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the dilution in ownership and voting power to be experienced by non-exercising stockholders;

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the dilutive effect the offering will have on the dividends per share we distribute subsequent to completion of the offering;

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the terms and expenses in connection with the offering relative to other alternatives for raising capital, including fees payable to the dealer managers;

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the size of the offering in relation to the number of shares outstanding;

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the market price of our common stock, both before and after the announcement of the rights offering;

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the general condition of the securities markets; and

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any impact on operating expenses associated with an increase in capital, including an increase in fees payable to GC Advisors.

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We cannot provide you any assurance of the amount of dilution, if any, that a stockholder will experience, that the current offering will be successful, or that by increasing the size of our available equity capital, our aggregate expenses and, correspondingly, our expense ratio will be lowered. In addition, the management fee we pay to GC Advisors is based upon our gross assets, which could include proceeds from this offering invested in the securities of portfolio companies, so we expect the management fee payable to GC Advisors to increase as a result of the current offering.

We have retained Wells Fargo Securities, LLC, the lead dealer manager for this offering, and J.P. Morgan Securities LLC and Morgan Stanley & Co. LLC to provide us with marketing and soliciting services relating to this offering, including assistance with respect to the structure, timing and terms of the offer. Our board of directors considered current secondary market trading conditions, using a fixed pricing versus variable pricing mechanism, the benefits and drawbacks of conducting a non-transferable versus a transferable rights offering, the effect on us if this offering is not fully subscribed, the experience of Wells Fargo Securities, LLC in conducting rights offerings, and the inclusion of an over-subscription privilege. There can be no assurances that the offering will be successful.

Although we have no present intention to do so, we have the ability, in the future and in our discretion, to choose to make additional rights offerings from time to time for a number of shares and on terms which could be similar to or different from this offering, provided that our board of directors must determine that each subsequent rights offering is in the best interest of our stockholders. Any such future rights offering will be made in accordance with the 1940 Act.

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Terms of the Offering

We issued to record date stockholders transferable rights to subscribe for up to approximately 33,451,902 shares of our common stock. Each record date stockholder was issued one transferable right for each four shares of our common stock owned on the record date. The rights entitle each holder, or rights holder, to acquire at the subscription price one share for every right held, which we refer to as the primary subscription right. Rights may be exercised at any time during the subscription period, which commences on April 9, 2020, the date following the record date, and ends at 5:00 p.m., New York City time, on May 6, 2020, unless extended by us in our sole discretion, the expiration date.

The rights are transferable and are traded on the Nasdaq Global Select Market under the symbol “GBDCR”. Our common stock is listed on the Nasdaq Global Select Market under the symbol “GBDC”. Rights holders who are not record date stockholders can purchase shares as described above, which we refer to as the primary subscription right, and are entitled to subscribe for shares pursuant to the over-subscription privilege (as described below). Non-record date rights holders who purchase shares pursuant to the primary subscription right or the over-subscription right, together with record date stockholders who purchase shares, are hereinafter referred to as participating rights holders. The rights are evidenced by subscription certificates which have been mailed to stockholders, except as discussed below under “— Foreign Stockholders.” We will not issue fractional rights.

Shares for which there is no subscription during the primary subscription will be offered, by means of the over-subscription privilege, first to record date stockholders who fully exercise the rights issued to them pursuant to this offering and who wish to acquire more than the number of shares they are entitled to purchase pursuant to the exercise of their rights. In addition, any non-record date rights holder who exercises rights is entitled to subscribe for remaining shares that are not otherwise subscribed for by record date stockholders. Shares acquired pursuant to the over-subscription privilege are subject to certain limitations and pro rata allocations. See “— Over-Subscription Privilege” below.

For purposes of determining the number of shares a record date stockholder can acquire pursuant to the offer, broker-dealers, trust companies, banks or others whose shares are held of record by Cede or by any other depository or nominee have been deemed to be the holders of the rights that are issued to Cede or the other depository or nominee on their behalf.

There is no minimum number of rights which must be exercised in order for the offering to close.

Over-Subscription Privilege

Shares not subscribed for by rights holders, which we refer to as remaining shares, will be offered, by means of the over-subscription privilege, first to record date stockholders (other than affiliates of Golub Capital) who have fully exercised the rights issued to them and who wish to acquire more than the number of shares they are entitled to purchase pursuant to the primary subscription rights. Rights holders should indicate on the subscription certificate that they submit with respect to the exercise of the rights issued to them how many additional shares they are willing to acquire pursuant to the over-subscription privilege. If there are sufficient remaining shares, all rights holders’ over-subscription requests will be honored in full. If record date stockholders’ (other than affiliates of Golub Capital) requests for shares pursuant to the over-subscription privilege exceed the remaining shares available, the available remaining shares will be allocated pro-rata among rights holders (other than affiliates of Golub Capital) who over-subscribe based on the number of shares held on the record date. The percentage of remaining shares each over-subscribing stockholder can acquire will be rounded down to result in delivery of whole shares. The allocation process could involve a series of allocations to assure that the total number of remaining shares available for over-subscriptions is distributed on a pro-rata basis. The formula to be used in allocating the remaining shares is as follows:

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However, if this pro-rata allocation results in any holder being allocated a greater number of shares than the holder subscribed for pursuant to the exercise of the over-subscription privilege, then such holder will be allocated only such number of shares pursuant to the over-subscription privilege as such holder subscribed for.

Any rights holder other than a record date stockholder who exercises rights is entitled to subscribe for remaining shares that are not otherwise over-subscribed for by record date stockholders (other than affiliates of Golub Capital). These non-record date rights holders should indicate, in the subscription certificate submitted with respect to the exercise of any rights, how many shares they are willing to acquire pursuant to the over-subscription privilege. There can be no assurance that non-record date rights holders will receive shares pursuant to the over-subscription privilege.

If sufficient remaining shares are available after the over-subscription privileges for the record date stockholders have been allotted, then all over-subscriptions by non-record date rights holders (other than affiliates of Golub Capital) will be honored in full. If the remaining shares are insufficient to permit such allocation, the remaining shares will be allocated pro-rata among the non-record date rights holders (other than affiliates of Golub Capital) who wish to exercise their over-subscription privilege, based on the number of rights held by such rights holders on the expiration date; provided, however, that if this pro-rata allocation results in any holder being allocated a greater number of shares than the holder subscribed for pursuant to the exercise of the over-subscription privilege, then such holder will be allocated only such number of shares pursuant to the over-subscription privilege as such holder subscribed for. The formula to be used in allocating the shares available to non-record date rights holders (other than affiliates of Golub Capital) exercising their over-subscription privilege is as follows:

Banks, brokers, trustees and other nominee holders of rights will be required to certify to the subscription agent, before any over-subscription privilege can be exercised with respect to any particular beneficial owner, as to the aggregate number of rights exercised pursuant to the primary subscription and the number of shares subscribed for pursuant to the over-subscription privilege by such beneficial owner and that such beneficial owner’s primary subscription was exercised in full. We will not offer or sell in connection with the offer any rights that are not subscribed for pursuant to the primary subscription or the over-subscription privilege.

Certain affiliates of Golub Capital LLC, an affiliate of GC Advisors, have indicated that they intend to over-subscribe and to make a total investment of up to $125 million in shares of our common stock pursuant to exercise of the primary subscription and/or the over-subscription privilege. As of March 31, 2020, these affiliates of Golub Capital LLC held an aggregate of more than 12.0 million shares of our common stock. Any over-subscription by affiliates of Golub Capital LLC will be effected only after the pro rata allocation of shares to (1) record date holders (other than affiliates of Golub Capital LLC) who fully exercise all rights issued to them and (2) any non-record date rights holder (other than an affiliate of Golub Capital LLC) who exercises rights.

Subscription Price

The subscription price for the shares to be issued pursuant to the offer will be 92.5% of the volume-weighted average of the sales prices of our shares of common stock on the Nasdaq Global Select Market for the five consecutive trading days ending on the expiration date of the offering. See “— Payment for Shares” below. Because the subscription price will be determined on the expiration date, rights holders will generally not know the subscription price at the time of exercise and will be required initially to pay for both the shares subscribed for pursuant to their primary subscription rights and, if eligible, any additional

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shares subscribed for pursuant to the over-subscription privilege at the estimated subscription price of $11.45 per share. Rights holders who exercise their rights will have no right to rescind their subscriptions after receipt of their completed subscription certificates together with payment for shares or a notice of guaranteed delivery by the subscription agent.

Expiration of the Offer

The offer will expire on the expiration date. The rights will expire on the expiration date of the rights offering and cannot be exercised thereafter.

Our board of directors, or a committee thereof, could determine to extend the subscription period, and thereby postpone the expiration date, to the extent our board of directors, or a committee thereof, determines that doing so is in the best interest of our stockholders. For example, our board of directors could elect to extend the subscription period in the event there is substantial instability or volatility in the trading price of our common stock or the rights on the Nasdaq Global Select Market at or near the expiration date, or if any event occurs which causes trading to cease or be suspended on the Nasdaq Global Select Market or the financial markets generally. The foregoing are not the only circumstances under which this offering can be extended, and our board of directors is free to extend the subscription period at its discretion, provided it determines that doing so is in the best interests of our stockholders.

Any extension of the offer will be followed as promptly as practicable by announcement thereof, and in no event later than 9:00 a.m., New York City time, on the next business day following the previously scheduled expiration date. Without limiting the manner in which we could choose to make such announcement, we will not, unless otherwise required by law, have any obligation to publish, advertise or otherwise communicate any such announcement other than by issuing a press release or such other means of announcement as we deem appropriate.

Dilutive Effects

Any stockholder who chooses not to participate in the offering should expect to own a smaller interest in us upon completion of the offering. The offering will dilute the ownership interest and voting power of stockholders who do not fully exercise their primary subscription rights. Further, because the net proceeds per share from the offering will likely be lower than our then-current NAV per share, the offering will likely reduce our NAV per share. The amount of dilution, if any, that a stockholder experiences could be substantial. See “Dilution.”

The transferable feature of the rights affords non-participating stockholders the potential of receiving cash payment upon the sale of rights, receipt of which could be viewed as partial compensation for the dilution of their interests.

Amendments and Waivers; Termination

We reserve the right to amend the terms and conditions of this offering, whether the amended terms are more or less favorable to you. We will comply with all applicable laws, including the federal securities laws, in connection with any such amendment.

We will decide all questions as to the validity, form and eligibility (including times of receipt, beneficial ownership and compliance with other procedural matters) in our sole discretion, and our determination shall be final and binding. The acceptance of subscription certificates and the subscription price also will be determined by us. Alternative, conditional or contingent subscriptions will not be accepted. We reserve the right to reject any exercise if such exercise is not in accordance with the terms of the offering or not in proper form or if the acceptance thereof or the issuance of shares of our common stock thereto could be deemed unlawful. We, in our sole discretion, have the ability to waive any defect or irregularity, or permit a defect or irregularity to be corrected within such time as we determine, or reject the purported exercise of any right. Subscriptions will not be deemed to have been received or accepted until all irregularities have been waived or cured within such time as we determine in our sole discretion. We will not be under any duty to give notification of any defect or irregularity in connection with the submission of subscription certificates or incur any liability for failure to give such notification.

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We have the ability to terminate this offering at any time prior to delivery of the rights or the shares of our common stock offered hereby by giving oral or written notice thereof to the subscription agent and making a public announcement thereof. In addition, the dealer managers have the right to terminate the dealer management agreement. If the offering is terminated, all rights will expire without value, and we will promptly arrange for the refund, without interest, of all funds received from holders of rights. All monies received by the subscription agent in connection with the offering are being held by the subscription agent, on our behalf, in a segregated interest-bearing account at a negotiated rate. All such interest shall be payable to us even if we determine to terminate the offering and return your subscription payment. In addition, no amounts paid to acquire rights on the Nasdaq Global Select Market or otherwise will be returned.

Information Agent and Subscription Agent

Broadridge Corporate Issuer Solutions, Inc. (“Broadridge”) is acting as the information agent and the subscription agent in connection with the offering. The information agent and subscription agent will receive for its services a fee estimated to be approximately $10,000 plus reimbursement of all out-of-pocket expenses related to the offering. Broadridge can be contacted at the below address:

Completed subscription certificates must be sent together with full payment of the estimated subscription price for all shares subscribed for in the primary subscription and pursuant to the over-subscription privilege to the subscription agent by one of the methods described below. We will accept only properly completed and duly executed subscription certificates actually received at any of the addresses listed below, on or prior to the expiration date or by the close of business on the second business day after the expiration date of the rights offering following timely receipt of a notice of guaranteed delivery. See “— Payment for Shares” below. In this prospectus supplement, close of business means 5:00 p.m., New York City time, on the relevant date.

Delivery to an address other than the addresses listed above will not constitute valid delivery.

Any questions or requests for assistance concerning the method of subscribing for shares or for additional copies of this prospectus supplement or subscription certificates or notices of guaranteed delivery can be directed to the information agent at its telephone number and address listed below:

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Stockholders can also contact their broker-dealers or nominees for information with respect to the offer.

Methods for Exercising Rights

Rights are evidenced by subscription certificates that, except as described below under “— Foreign Stockholders,” have been mailed to record date stockholders or, if a record date stockholder’s shares are held by Cede or any other depository or nominee on their behalf, to Cede or such depository or nominee. Rights can be exercised by completing and signing the subscription certificate that accompanies this prospectus supplement and mailing it in the envelope provided, or otherwise delivering the completed and duly executed subscription certificate to the subscription agent, together with payment in full for the shares at the estimated subscription price by the expiration date. Rights can also be exercised by contacting your broker, trustee or other nominee, who can arrange, on your behalf, to guarantee delivery of payment and delivery of a properly completed and duly executed subscription certificate pursuant to a notice of guaranteed delivery by the close of business on the second business day after the expiration date. A fee may be charged for this service. Completed subscription certificates and related payments must be received by the subscription agent on or prior to the expiration date (unless payment is effected by means of a notice of guaranteed delivery as described below under “— Payment for Shares”) at the offices of the subscription agent at the address set forth above. Fractional shares will not be issued upon the exercise of rights.

Exercise of the Over-Subscription Privilege

Record date stockholders who fully exercise all rights issued to them can participate in the over-subscription privilege by indicating on their subscription certificate the number of shares they are willing to acquire. If sufficient remaining shares are available after the primary subscription, all over-subscriptions will be honored in full; otherwise remaining shares will be allocated as described under “— Over-Subscription Privilege” above.

Record Date Stockholders Whose Shares Are Held By a Nominee

Record date stockholders whose shares are held by a nominee, such as a bank, broker-dealer or trustee, must contact that nominee to exercise their rights. In that case, the nominee will complete the subscription certificate on behalf of the record date stockholder and arrange for proper payment by one of the methods set forth under “— Payment for Shares” below.

Nominees

Nominees, such as brokers, trustees or depositories for securities, who hold shares for the account of others, should notify the respective beneficial owners of the shares as soon as possible to ascertain the beneficial owners’ intentions and to obtain instructions with respect to the rights. If the beneficial owner so instructs, the nominee should complete the subscription certificate and submit it to the subscription agent with the proper payment as described under “— Payment for Shares” below.

All questions as to the validity, form, eligibility (including times of receipt and matters pertaining to beneficial ownership) and the acceptance of subscription forms and the subscription price will be determined by us, which determinations will be final and binding. No alternative, conditional or contingent subscriptions will be accepted. We reserve the right to reject any or all subscriptions not properly submitted or the acceptance of which would, in the opinion of our counsel, be unlawful.

We reserve the right to reject any exercise if such exercise is not in accordance with the terms of this rights offering or not in proper form or if the acceptance thereof or the issuance of shares of our common stock thereto could be deemed unlawful. We reserve the right to waive any deficiency or irregularity with

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respect to any subscription certificate. Subscriptions will not be deemed to have been received or accepted until all irregularities have been waived or cured within such time as we determine in our sole discretion. We will not be under any duty to give notification of any defect or irregularity in connection with the submission of subscription certificates or incur any liability for failure to give such notification.

Foreign Stockholders

Subscription certificates will not be mailed to registered foreign stockholders. Registered foreign stockholders will receive written notice of this offering. The subscription agent will hold the rights to which those subscription certificates relate for these stockholders’ accounts until instructions are received to exercise the rights and such stockholders establish to the satisfaction of the subscription agent that they are permitted to exercise their subscription rights under applicable law. In addition, such stockholders must take all other steps that are necessary to exercise their subscription rights on or prior to the date required for participation in the rights offering. If no instructions have been received by the expiration date, or if those rights are not otherwise purchased or sold prior to the expiration of the rights offering, they will expire.

Payment for Shares

Participating rights holders can choose between the following methods of payment:

(1)

A participating rights holder can send the subscription certificate together with payment for the shares acquired in the primary subscription and any additional shares subscribed for pursuant to the over-subscription privilege to the subscription agent based on an estimated subscription price per share of  $11.45, which is estimated on the basis of 92.5% of the volume-weighted average of the sales prices of our shares of common stock on the Nasdaq Global Select Market for the five consecutive trading days ending on March 30, 2020. To be accepted, the estimated payment, together with a properly completed and executed subscription certificate, must be received by the subscription agent at one of the subscription agent’s offices set forth above, on or prior to the expiration date.

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(2)

A participating rights holder can request a Eligible Guarantor Institution as that term is defined in Rule 17Ad-15 under the Exchange Act to send a notice of guaranteed delivery by facsimile or otherwise guaranteeing delivery of  (i) payment of the full estimated subscription price for the shares subscribed for in the primary subscription and any additional shares subscribed for pursuant to the over-subscription privilege and (ii) a properly completed and duly executed subscription certificate. The subscription agent will not honor a notice of guaranteed delivery unless a properly completed and duly executed subscription certificate and full payment for the shares is received by the subscription agent on or prior to close of business on the second business day after the expiration date.

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Participating rights holders will have no right to rescind their subscription after receipt of their payment for shares or a notice of guaranteed delivery by the subscription agent.

All payments by a participating rights holder must be in U.S. dollars by check or bank draft drawn on a bank or branch located in the United States and payable to Broadridge Corporate Issuer Solutions, Inc., as subscription agent. A participating rights holder could also wire the transfer of immediately available funds directly to the account maintained by Broadridge Corporate Issuer Solutions, Inc. at U.S. Bank, 800 Nicollet Mall, Minneapolis, Minnesota 55402 Routing Number 123000848, Swift Code USBKUS44IMT, Accounting Number 153910728465, Beneficiary account name: Broadridge, as subscription agent, for purposes of accepting subscriptions in this rights offering, with reference to the Golub Capital Subscription Offering, the rights holder’s name and the for further credit name Golub Capital BDC, Inc. The subscription agent will deposit all funds received by it prior to the final payment date into a segregated account pending pro-ration and distribution of the shares. If the offering is terminated, we will promptly arrange for the refund, without interest, of all funds received from holders of rights.

The method of delivery of subscription certificates and payment of the subscription price to us is at the election and risk of the participating rights holders, but if sent by mail it is recommended that such certificates and payments be sent by registered mail, properly insured, with return receipt requested, and that a sufficient

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number of days be allowed to ensure delivery to the subscription agent and clearance of payment on prior to the expiration date or the date guaranteed payments are due under a notice of guaranteed delivery (as applicable). Because uncertified personal checks can take at least five business days to clear, you are strongly urged to pay, or arrange for payment, by means of certified or cashier’s check or money order.

As soon as practicable following the expiration date, the subscription agent will send to each participating rights holder (or, if rights are held by Cede or any other depository or nominee, to Cede or such other depository or nominee) a statement or certificate showing (i) the number of shares purchased pursuant to the primary subscription; (ii) the number of shares, if any, acquired pursuant to the over-subscription privilege; (iii) the per share and total purchase price for such shares; and (iv) any additional amount payable to us by the participating rights holder or any excess to be refunded by us to the participating rights holder, in each case based on the subscription price as determined on the expiration date. If any participating rights holder, if eligible, exercises his or her right to acquire shares pursuant to the over-subscription privilege, any excess payment which would otherwise be refunded to him or her will be applied by us toward payment for shares acquired pursuant to the exercise of the over-subscription privilege. Any additional payment required from a participating rights holder must be received by the subscription agent within ten business days after the confirmation date. Any excess payment to be refunded by us to a participating rights holder will be mailed by the subscription agent to the rights holder as promptly as practicable. No interest will be paid on any amounts refunded.

Whichever of the two methods described above is used, issuance of the shares purchased is subject to collection of checks and actual payment. If a participating rights holder who subscribes for shares pursuant to the primary subscription or over-subscription privilege does not make payment of any amounts due by the expiration date, the date guaranteed payments are due under a notice of guaranteed delivery or within ten business days of the confirmation date, as applicable, the subscription agent reserves the right to take any or all of the following actions: (1) reallocate the shares to other participating rights holders in accordance with the over-subscription privilege; (2) apply any payment actually received by it from the participating rights holder toward the purchase of the greatest whole number of shares which could be acquired by such participating rights holder upon exercise of the primary subscription and/or the over-subscription privilege; and/or (3) exercise any and all other rights or remedies to which it may be entitled, including the right to set off against payments actually received by it with respect to such subscribed for shares.

All questions concerning the timeliness, validity, form and eligibility of any exercise of rights (including times of receipt, beneficial ownership and compliance with other procedural matters) will be determined by us in our sole discretion, which determinations will be final and binding. We in our sole discretion have the ability to waive any defect or irregularity, or permit a defect or irregularity to be corrected within such time as we determine, or reject the purported exercise of any right. Subscriptions will not be deemed to have been received or accepted until all irregularities have been waived or cured within such time as we determine in our sole discretion. The subscription agent is not under any duty to give notification of any defect or irregularity in connection with the submission of subscription certificates or incur any liability for failure to give such notification.

Notice of Net Asset Value Decline

We will suspend the offer until we amend this prospectus supplement if, subsequent to the date of this prospectus supplement, our NAV declines more than 10% from our NAV as of that date. Accordingly, the expiration date would be extended and we would notify record date stockholders of the decline.

Delivery of Stock Certificates and Book-Entry

Stock certificates will not be issued for shares of our common stock offered in the offering. Stockholders who are record owners will have the shares they acquire credited to their account with our transfer agent. Participants in our dividend reinvestment plan will have any shares that they acquire pursuant to the offer credited to their stockholder dividend reinvestment accounts in the plan. Stockholders whose shares are held of record by Cede or by any other depository or nominee on their behalf or their broker-dealers’ behalf will have any shares that they acquire credited to the account of Cede or the other depository or nominee.

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Federal Income Tax Consequences of the Offer

For federal income tax purposes, neither the receipt nor the exercise of the rights by record date stockholders will result in taxable income to such stockholders, and no loss will be realized if the rights expire without exercise.

A record date stockholder’s basis in a right will be zero (and such stockholder’s basis in its shares with respect to which such right was distributed will remain unchanged) unless either (1) the fair market value of the right on the date of distribution is 15% or more of the fair market value of the shares with respect to which the right was distributed or (2) the record date stockholder elects, in his or her federal income tax return for the taxable year in which the right is received, to allocate part of the basis of the shares to the right. If either of clauses (1) or (2) is applicable, then if the right is exercised, the record date stockholder will allocate his or her basis in the shares with respect to which the right was distributed between the shares and the right in proportion to the fair market values of each on the date of distribution.

The holding period of a right received by a record date stockholder includes the holding period of the shares with regard to which the right is issued. If the right is exercised, the holding period of the shares acquired begins on the date the right is exercised.

If a right is sold, a gain or loss will be recognized by the rights holder in an amount equal to the difference between the basis of the right sold and the amount realized on its disposition. Such gain or loss will be a capital gain or loss (assuming the right was held as a capital asset) and long-term capital gain or loss if the right was deemed to be held for more than one year.

A record date stockholder’s basis for determining gain or loss upon the sale of a share acquired upon the exercise of a right will be equal to the sum of the record date stockholder’s basis in the right, if any, and the subscription price per share. A record date stockholder’s gain or loss recognized upon a sale of a share acquired upon the exercise of a right will be capital gain or loss (assuming the share was held as a capital asset at the time of sale) and will be long-term capital gain or loss if the share is held for more than one year.

The foregoing is a general summary of the material U.S. federal income tax consequences of the offer under the provisions of the Code and Treasury regulations in effect as of the date of this prospectus supplement that are generally applicable to record date stockholders who are United States persons within the meaning of the Code, and does not address any foreign, state or local tax consequences. The Code and Treasury regulations are subject to change or differing interpretations by legislative or administrative action, which may be retroactive. This summary does not take into account any considerations that may relate to special classes of stockholders. Participating rights holders should consult their tax advisors regarding specific questions as to foreign, federal, state or local taxes.

ERISA Considerations

Stockholders who are employee benefit plans subject to the Employee Retirement Income Security Act of 1974, or ERISA, (including corporate savings and 401(k) plans), Keogh or H.R. 10 plans of self-employed individuals and individual retirement accounts should be aware that additional contributions of cash to a retirement plan (other than rollover contributions or trustee-to-trustee transfers from other retirement plans) in order to exercise rights would be treated as contributions to the retirement plan and, when taken together with contributions previously made, may result in, among other things, excise taxes for excess or nondeductible contributions. In the case of retirement plans qualified under Section 401(a) of the Code and certain other retirement plans, additional cash contributions could cause the maximum contribution limitations of Section 415 of the Code or other qualification rules to be violated. It may also be a reportable distribution and there may be other adverse tax and ERISA consequences if rights are sold or transferred by a retirement plan.

Retirement plans and other tax exempt entities, including governmental plans, should also be aware that if they borrow in order to finance their exercise of rights, they may become subject to the tax on unrelated business taxable income under Section 511 of the Code. If any portion of an individual retirement account is used as security for a loan, the portion so used is also treated as distributed to the IRA depositor. ERISA contains fiduciary responsibility requirements, and ERISA and the Code contain

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prohibited transaction rules that may impact the exercise of rights. Due to the complexity of these rules and the penalties for noncompliance, retirement plans should consult with their counsel and other advisers regarding the consequences of their exercise of rights under ERISA and the Code.

Distribution Arrangements

Wells Fargo Securities, LLC, J.P. Morgan Securities LLC and Morgan Stanley & Co. LLC, each a broker-dealer and member of the Financial Industry Regulatory Authority, are acting as dealer managers for this offering. Under the terms and subject to the conditions contained in the dealer management agreement, the dealer managers are providing marketing services in connection with this offering and soliciting the exercise of rights and participation in the over-subscription privilege. This offering is not contingent upon any number of rights being exercised. We have agreed to pay the dealer managers a fee for marketing and soliciting services equal to 2% of the subscription price per share for each share issued pursuant to exercise of the primary subscription and/or the over-subscription privilege other than affiliates of Golub Capital LLC, an affiliate of GC Advisors, and 1% of the subscription price per share for each share issued to affiliates of Golub Capital LLC, provided that the dealer managers are permitted to waive certain of the amounts to which they are entitled.

The dealer managers will reallow to other broker-dealers that have executed and delivered a soliciting dealer agreement and have solicited the exercise of rights, solicitation fees up to 1% of the subscription price per share for each share issued pursuant to the exercise of rights as a result of their soliciting efforts, subject to a maximum fee based on the number of shares held by each broker-dealer through the Depository Trust Company on the record date. Fees will be paid by us to the broker-dealer designated on the applicable portion of the subscription certificates or, in the absence of such designation, to the dealer managers. We have agreed to reimburse the dealer managers for their out-of-pocket expenses, including the reasonable fees and disbursements of counsel for the dealer managers, in an amount up to $150,000 and to reimburse the fees and expenses of the dealer managers in connection with review of this offering by FINRA in an amount up to $10,000. In addition, the dealer managers have agreed to reimburse us for certain of our out-of-pocket expenses in certain circumstances.

We, GC Advisors and the Administrator have agreed to indemnify the dealer managers for, or contribute to losses arising out of, certain liabilities, including liabilities under the Securities Act. The dealer manager agreement also provides that the dealer managers will not be subject to any liability to us in rendering the services contemplated by the dealer manager agreement except for any act of bad faith, willful misfeasance, or gross negligence of the dealer managers or reckless disregard by the dealer managers of their obligations and duties under the dealer manager agreement. We have also agreed not to directly or indirectly sell, offer to sell, enter into any agreement to sell, or otherwise dispose of, any of our equity or equity related securities or securities convertible into such securities, other than the rights, the shares and the common stock issued in connection with the reinvestment of dividends or distributions, for a period of 60 days from the date of the dealer manager agreement without the prior consent of Wells Fargo Securities, LLC.

The principal business address of Wells Fargo Securities, LLC is 550 South Tryon Street, Charlotte, NC 28202.

This offering is being conducted in compliance with Rule 5110 of the Conduct Rules of the Financial Industry Regulatory Authority.

Additional Dealer Manager Compensation

The dealer managers and/or their affiliates have from time to time performed and likely will, in the future perform various commercial banking, financial advisory and investment banking services for us and our affiliates for which they have received or will receive customary compensation. In particular, the dealer managers or their affiliates execute transactions with us and our affiliates or any of our portfolio companies. In addition, certain of the dealer managers or their affiliates act as arranger, underwriter or placement agent for companies whose securities are sold to or whose loans are syndicated to the us or our

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affiliates. The dealer managers or their affiliates also trades in our securities, securities of our portfolio companies or other financial instruments related thereto for their own accounts or for the account of others and extends loans or financing directly or through derivative transactions to us and our affiliates or any of the portfolio companies.

Wells Fargo Bank, N.A., an affiliate of Wells Fargo Securities, LLC, serves as lender and collateral agent under the Credit Facility, as lender under the WF Credit Facility, as collateral agent and as collateral custodian under the DB Credit Facility, as collateral agent, account bank and collateral custodian under the MS Credit Facility II, as lender under the SLF Credit Facility and as lender under the GCIC SLF Credit Facility. Wells Fargo Securities, LLC serves as administrative agent under the WF Credit Facility and as administrative agent under the SLF Credit Facility. Certain of our subsidiaries paid Wells Fargo Securities, LLC structuring and placement fees for its services in connection with the initial structuring and/or subsequent amendments to the 2014 Debt Securitization and a prior debt securitization of GBDC. Before we acquired the GCIC 2018 Issuer as part of the Merger, the GCIC 2018 Issuer paid Wells Fargo Securities, LLC structuring and placement fees for its services in connection with the initial structuring of the GCIC 2018 Debt Securitization. Morgan Stanley Bank, N.A., an affiliate of Morgan Stanley & Co. LLC, serves as lender under the MS Credit Facility. Morgan Stanley Senior Funding, Inc., an affiliate of Morgan Stanley & Co. LLC, serves as administrative agent under each of the MS Credit Facility and the MS Credit Facility II. The 2018 Issuer paid Morgan Stanley & Co. LLC structuring and placement fees for its services in connection with the structuring of the 2018 Debt Securitization.

Prior to the expiration of the offering, the dealer managers can independently offer for sale shares, including shares acquired through purchasing and exercising the rights, at prices it sets. The dealer managers could realize profits or losses independent of any fees described in this prospectus supplement.

Certain Effects of this Offering

GC Advisors will benefit from this offering because a portion of the investment advisory fee we pay to GC Advisors is based on our gross assets. See “Item 1. Business — Management Agreements — Investment Advisory Agreement — Management Fee” included in our most recent Annual Report on Form 10-K. It is not possible to state precisely the amount of additional compensation GC Advisors will receive as a result of this offering because it is not known how many shares will be subscribed for and because a substantial portion of the proceeds of the offer are expected to be used to repay outstanding indebtedness. However, assuming (1) all rights are exercised, (2) the average value of our gross assets, excluding proceeds from this offering, remains at approximately $4.695 billion, (3) an estimated subscription price per share of  $11.45, which represents 92.5% of the volume-weighted average of the sales prices of our shares of common stock on the Nasdaq Global Select Market for the five consecutive trading days ending on March 30, 2020, and (4) all of the net proceeds from the offer are invested in additional portfolio companies, and after giving effect to dealer manager fees and other expenses related to this offering, GC Advisors would receive additional annualized base advisory fees of approximately $5.2 million, and the amount of the administrative fee received by the Administrator would not change. Two of our directors who voted to authorize this offering are interested persons of GC Advisors. The other four directors who approved this offering are not affiliated with GC Advisors.

As a result of the terms of this offering, stockholders who do not fully exercise their rights will own, upon completion of this offering, a smaller proportional interest in us than they owned prior to the offer, including with respect to voting rights. To the affiliates of Golub Capital exercise their oversubscription privileges and receive an allocation of shares, their respective ownership interests will increase.

In addition, because the subscription price per share will likely be less than the NAV per share, based on our current market price, the offer will likely result in an immediate dilution of NAV per share for all of our stockholders. If the subscription price per share is substantially less than the then-current NAV per share, such dilution could be substantial. Any such dilution will disproportionately affect non-exercising stockholders. If the subscription price is less than our then-current NAV per share, then all stockholders will experience a decrease in the NAV per share held by them, irrespective of whether they exercise all or any portion of their rights. This offering will also cause dilution in the dividends per share we are able to distribute subsequent to completion of the offering. See “Dilution.”

S-43

Sale of Rights

The Rights are Transferable until the Trading Day Immediately Preceding the Expiration Date

The subscription rights issued in this offering are transferable. If you are a beneficial owner of shares of our common stock that are held of record in the name of a broker, bank or other nominee, you should ask that entity to effect the sale of your rights or the purchase of other rights that may be available. If you are a stockholder of record, whether you hold certificates of our common stock directly or in book-entry form with our transfer agent, you will need to engage a broker to effect the transactions for you.

The rights are traded on the Nasdaq Global Select Market under the symbol “GBDCR.” While the dealer managers will use their best efforts to ensure that an adequate trading market for the rights will exist, no assurance can be given that a market for the rights will develop. The rights are transferable and are expected to continue trading until and including May 5, 2020 (or if the offering is extended, until the trading day immediately prior to the extended expiration date). Rights holders are encouraged to contact their broker-dealer, bank, trustee or other nominees for more information about trading of the rights.

Other Transfers

The rights evidenced by a subscription certificate can be transferred in whole by endorsing the subscription certificate for transfer in accordance with the accompanying instructions. A portion of the rights evidenced by a single subscription certificate can be transferred by delivering to the subscription agent a subscription certificate properly endorsed for transfer, with instructions to register such portion of the rights evidenced thereby in the name of the transferee and to issue a new subscription certificate to the transferee evidencing such transferred rights. In such event, a new subscription certificate evidencing the balance of the rights, if any, will be issued to the stockholder or, if the stockholder so instructs, to an additional transferee. The signature on the subscription certificate must correspond to the name as written upon the face of the subscription certificate, without alteration or enlargement, or any change. A signature guarantee must be provided by an Eligible Guarantor Institution, subject to the standards and procedures adopted by us.

Stockholders wishing to transfer all or a portion of their rights should allow at least five business days prior to the expiration date of the offering for (i) the transfer instructions to be received and processed by the subscription agent; (ii) a new subscription certificate to be issued and transmitted to the transferee or transferees with respect to transferred rights, and to the transferor with respect to retained rights, if any; and (iii) the rights evidenced by such new subscription certificate to be exercised or sold by the recipients thereof. Neither we nor the subscription agent nor the dealer managers shall have any liability to a transferee or transferor of rights if subscription certificates are not received in time for exercise prior to the expiration date of the offering or sale prior to the day immediately preceding the expiration date of the offering (or, if the offering is extended, the extended expiration date).

Except for the fees charged by the subscription agent, which will be paid by us, all commissions, fees and other expenses (including brokerage commissions and transfer taxes) incurred or charged in connection with the purchase, sale or exercise of rights will be for the account of the transferor of the rights, and none of those commissions, fees or expenses will be paid by us, the subscription agent or the dealer managers.

The rights are eligible for transfer through, and the exercise of the primary subscription rights and the over-subscription privilege are being effected through, the facilities of the Depository Trust Company.

S-44

LEGAL MATTERS

Certain legal matters regarding the securities offered by this prospectus supplement will be passed upon for us by Dechert LLP, Boston, MA. Dechert LLP has from time to time represented GC Advisors and the dealer managers on unrelated matters. Certain legal matters in connection with the securities offered hereby will be passed upon for the dealer managers by Proskauer Rose LLP, Washington, D.C.

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

The consolidated financial statements of Golub Capital BDC, Inc. and its consolidated subsidiaries at September 30, 2019 and 2018 and for each of the three years in the period ended September 30, 2019 and the effectiveness of Golub Capital BDC, Inc. and its consolidated subsidiaries’ internal control over financial reporting as of September 30, 2019 included in our most recent Annual Report on Form 10-K have been audited by Ernst & Young LLP, an independent registered public accounting firm located at 155 North Wacker Drive, Chicago, IL 60606, as set forth in their respective reports thereon in our Annual Report on Form 10-K, and are incorporated by reference in reliance upon such reports given on the authority of Ernst & Young LLP as experts in accounting and auditing.

INCORPORATION BY REFERENCE

This prospectus supplement is part of a registration statement that we have filed with the SEC. Pursuant to the Small Business Credit Availability Act, we are allowed to “incorporate by reference” the information that we file with the SEC, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to be part of this prospectus supplement, and later information that we file with the SEC will automatically update and supersede this information.

We incorporate by reference the documents listed below and any future filings (including those made after the date of the filing of this prospectus supplement) we will make with the SEC under Sections 13(a), 13(c), 14, or 15(d) of the Exchange Act, subsequent to the date of this prospectus supplement until all of the securities offered by this prospectus supplement and the accompanying prospectus have been sold or we otherwise terminate the offering of these securities; provided, however, that information “furnished” under Item 2.02 or Item 7.01 of Form 8-K or other information “furnished” to the SEC, which is not deemed filed, is not incorporated by reference (unless specifically set forth in such filing):

•

our Annual Report on Form 10-K for the fiscal year ended September 30, 2019, filed with the SEC on November 25, 2019;

​

•

our Quarterly Report on Form 10-Q for the fiscal quarter ended December 31, 2019, filed with the SEC on February 10, 2020;

​

•

our Definitive Proxy Statement on Schedule 14A, filed with the SEC on December 16, 2019; and

​

•

our Current Reports on Form 8-K (other than information furnished rather than filed in accordance with SEC rules) filed with the SEC on October 16, 2019, October 31, 2019, January 6, 2020, February 6, 2020 and March 26, 2020.

​

To obtain copies of these filings, see “Available Information.”

S-45

AVAILABLE INFORMATION

We have filed with the SEC a registration statement on Form N-2, together with all amendments and related exhibits, under the Securities Act, with respect to the common stock offered by this prospectus supplement and the accompanying prospectus. The registration statement contains additional information about us and our common stock being offered by this prospectus supplement and the accompanying prospectus.

We file with or submit to the SEC annual, quarterly and current reports, proxy statements and other information meeting the informational requirements of the Exchange Act. The SEC maintains a website that provides access, free of charge, to reports, proxy and information statements and other information we file with the SEC at www.sec.gov. Copies of these reports, proxy and information statements and other information, as well as the registration statement and related exhibits and schedules, can be obtained, after paying a duplicating fee, by electronic request at the following e-mail address: publicinfo@sec.gov, or by writing the SEC’s Public Reference Section, 100 F Street, N.E., Washington, D.C. 20549-0102. We maintain a website at www.golubcapitalbdc.com and make all of our annual, quarterly and current reports, proxy statements and other publicly filed information available, free of charge, on or through our website. Information contained on our website is not incorporated into this prospectus supplement, and you should not consider information on our website to be part of this prospectus supplement. You can also obtain such information by calling us collect at (212) 750-6060 or by contacting us at 200 Park Avenue, 25th Floor, New York, New York 10166, Attention: Investor Relations or investorrelations@golubcapital.com.

S-46

Prospectus

$800,000,000

GOLUB CAPITAL BDC, INC.

Common Stock
Preferred Stock
Warrants
Subscription Rights
Debt Securities

We are an externally managed, closed-end, non-diversified management investment company that has elected to be regulated as a business development company under the Investment Company Act of 1940, as amended. Our investment objective is to generate current income and capital appreciation by investing primarily in one stop and other senior secured loans of U.S. middle-market companies. We may also selectively invest in second lien and subordinated loans of, and warrants and minority equity securities in U.S. middle-market companies.

GC Advisors LLC serves as our investment adviser. Golub Capital LLC serves as our administrator. GC Advisors LLC and Golub Capital LLC are affiliated with Golub Capital (as defined herein), a leading lender to middle-market companies that has over $30.0 billion of capital under management as of March 31, 2019.

We may offer, from time to time, in one or more offerings or series, together or separately, up to $800,000,000 of our common stock, preferred stock, warrants representing rights to purchase shares of our common stock, preferred stock or debt securities, subscription rights or debt securities, which we refer to, collectively, as the “securities.” We may sell our common stock through underwriters or dealers, “at-the-market” to or through a market maker into an existing trading market or otherwise directly to one or more purchasers or through agents or through a combination of methods of sale. The identities of such underwriters, dealers, market makers or agents, as the case may be, will be described in one or more supplements to this prospectus. The securities may be offered at prices and on terms to be described in one or more supplements to this prospectus, or any free writing prospectuses that we have authorized to use in connection with a specific offering. In the event we offer common stock, the offering price per share of our common stock exclusive of any underwriting commissions or discounts will not be less than the net asset value per share of our common stock at the time we make the offering except (1) in connection with a rights offering to our existing stockholders, (2) with the consent of the majority of our common stockholders and approval of our board of directors or (3) under such circumstances as the Securities and Exchange Commission, or the SEC, may permit. See “Risk Factors” included in, or incorporated by reference into, this prospectus, the applicable prospectus supplement and in the related free writing prospectuses that we have authorized for use in connection with a specific offering, and under similar headings in the other documents that are incorporated by reference into this prospectus for more information.

Our common stock is traded on The Nasdaq Global Select Market under the symbol “GBDC”. The last reported closing price for our common stock on June 25, 2019 was $17.63 per share. The net asset value of our common stock on March 31, 2019 (the last date prior to the date of this prospectus on which we determined net asset value) was $15.95 per share.

Shares of closed-end investment companies, including business development companies, frequently trade at a discount to their net asset value. If our shares trade at a discount to our net asset value, it will likely increase the risk of loss for purchasers in this offering. Investing in our securities involves a high degree of risk. Before buying any securities, you should read the discussion of the material risks of investing in our securities, including the risk of leverage, included in “Risk Factors” beginning on page 8 of this prospectus or otherwise incorporated by reference herein, and included or incorporated by reference into the applicable prospectus supplement and in any related free writing prospectuses that we have authorized for use in connection with a specific offering, and under similar headings in the other documents that are incorporated by reference into this prospectus.

This prospectus describes some of the general terms that may apply to an offering of our securities. We will provide the specific terms of these offerings and securities in one or more supplements to this prospectus. We may also authorize one or more free writing prospectuses to be provided to you in connection with these offerings. The prospectus supplement and any related free writing prospectus may also add, update, or change information contained in this prospectus. You should carefully read this prospectus, the applicable prospectus supplement, and any related free writing prospectus, and the documents incorporated by reference before you invest in our securities. We file annual, quarterly and current reports, proxy statements and other information about us with the SEC. We maintain a website at http://www.golubcapitalbdc.com and make all of our annual, quarterly and current reports, proxy statements and other publicly filed information available on or through our website. Information on our website is not incorporated into or a part of this prospectus or any related prospectus supplement or free writing prospectus. You may also obtain such information, free of charge, and make shareholder inquiries by contacting us at 666 Fifth Avenue, 18th Floor, New York, New York 10103, Attention: Investor Relations, or by calling us collect at (212) 750-6060. The SEC also maintains a website at http://www.sec.gov that contains such information.

We generally invest in securities that have been rated below investment grade by independent rating agencies or that would be rated below investment grade if they were rated. These securities, which may be referred to as “junk,” have predominantly speculative characteristics with respect to the issuer’s capacity to pay interest and repay principal. In addition, many of our debt investments have floating interest rates that reset on a periodic basis and typically do not fully pay down principal prior to maturity, which may increase our risk of losing part or all of our investment.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

This prospectus may not be used to consummate sales of securities unless accompanied by a prospectus supplement.

The date of this prospectus is June 26, 2019.

You should rely only on the information included or incorporated by reference in this prospectus, any prospectus supplement or in any free writing prospectus prepared by or on behalf of us or to which we have referred to you. We have not, and the underwriters have not, authorized any other person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not, and the underwriters are not, making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should not assume that the information included or incorporated by reference in this prospectus or any prospectus supplement or in any such free writing prospectus is accurate as of any date other than their respective dates. Our business, financial condition, results of operations, cash flows and prospects may have changed since that date.

TABLE OF CONTENTS

i

ABOUT THIS PROSPECTUS

This prospectus is part of an automatic registration statement that we have filed with the SEC using the “shelf” registration process as a “well-known seasoned issuer” as defined in Rule 405 under the Securities Act of 1933, as amended (the “Securities Act”). Under the shelf registration process, we may offer from time to time in one or more offerings, up to $800,000,000 of our common stock, preferred stock, warrants representing rights to purchase shares of our common stock, preferred stock or debt securities, subscription rights or debt securities on the terms to be determined at the time of the offering. We may sell our securities through underwriters or dealers, “at-the-market” to or through a market maker, into an existing trading market or otherwise directly to one or more purchasers or through agents or through a combination of methods of sale. The identities of such underwriters, dealers, market makers or agents, as the case may be, will be described in one or more supplements to this prospectus, or the free writing prospectuses that we have authorized for use in connection with a specific offering.

This prospectus provides you with a general description of the securities that we may offer. Each time we use this prospectus to offer securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering. We may also authorize one or more free writing prospectuses to be provided to you that may contain material information relating to that offering. The prospectus supplement or free writing prospectus may also add, update or change information contained in this prospectus or in the documents we have incorporated by reference into this prospectus. This prospectus, together with the applicable prospectus supplement, any related free writing prospectus, and the documents incorporated by reference into this prospectus and the applicable prospectus supplement will serve as the prospectus relating to the applicable offering. Before buying any of the securities being offered, please carefully read this prospectus, the applicable prospectus supplement, and any related free writing prospectus, together with the additional information described in the sections titled “Risk Factors” and “Available Information.”

This prospectus includes summaries of certain provisions contained in some of the documents described in this prospectus, but reference is made to the actual documents for complete information. All of the summaries are qualified in their entirety by the actual documents. Copies of some of the documents referred to herein have been filed, will be filed, or will be incorporated by reference as exhibits to the registration statement of which this prospectus is a part, and you may obtain copies of those documents as described in the section titled “Available Information.”

ii

​

1

2

3

4

​	Stockholder transaction expenses:	​	​	​	​	​	​	​
​	Sales load (as a percentage of offering price)	​	​	​	​	—%(1)	​	​
​	Offering expenses (as a percentage of offering price)	​	​	​	​	—%(2)	​	​
​	Dividend reinvestment plan expenses	​	​	None(3)​				​
​	Total stockholder transaction expenses (as a percentage of offering price)	​	​	​	​	—%	​	​
​	Annual expenses (as a percentage of net assets attributable to common stock):	​	​	​	​	​	​	​
​	Management fees	​	​	​	​	2.73%(4)	​	​
​	Incentive fees payable under the Investment Advisory Agreement	​	​	​	​	1.55%(5)	​	​
​	Interest payments on borrowed funds	​	​	​	​	4.40%(6)	​	​
​	Other expenses	​	​	​	​	0.60%(7)	​	​
​	Acquired fund fees and expenses	​	​	​	​	0.03%(8)	​	​
​	Total annual expenses	​	​	​	​	9.31%(9)	​	​

5

​

6

You would pay the following expenses on a $1,000 investment

​

​

1 year

​

​

3 years

​

​

5 years

​

​

10 years

​

Assuming a 5% annual return (assumes no return from net realized capital gains or net unrealized capital appreciation)

​

​

​

$

78

​

​

​

​

$

226

​

​

​

​

$

367

​

​

​

​

$

686

​

​

Assuming a 5% annual return (assumes return entirely from realized capital gains and thus subject to the capital gain incentive fee)

​

​

​

$

88

​

​

​

​

$

253

​

​

​

​

$

406

​

​

​

​

$

742

​

​

7

RISK FACTORS

Investing in our securities involves a number of significant risks. Before you invest in our securities, you should carefully consider various risks described in the section titled “Risk Factors” below, in the applicable prospectus supplement and any related free writing prospectus, and discussed in the section titled “Risk Factors” in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC, which are incorporated by reference into this prospectus in their entirety, together with other information in this prospectus, the documents incorporated by reference, and any free writing prospectus that we may authorize for use in connection with this offering. The risks set out in these documents are not the only risks we face. Additional risks and uncertainties not presently known to us or not presently deemed material by us may also impair our operations and performance. If any of these risks occur, our business, financial condition, results of operations and cash flows could be materially and adversely affected. In such case, our net asset value and the trading price of our common stock could decline, and you may lose all or part of your investment. The risk factors described in these documents are the principal risk factors associated with an investment in us as well as those factors generally associated with an investment company with investment objectives, investment policies, capital structure or trading markets similar to ours.

Risks Relating to Our Investments

The market price of our securities may fluctuate significantly.

The market price and liquidity of the market for our securities may be significantly affected by numerous factors, some of which are beyond our control and may not be directly related to our operating performance. These factors include:

•

significant volatility in the market price and trading volume of securities of business development companies or other companies in our sector, which are not necessarily related to the operating performance of the companies;

​

•

changes in regulatory policies, accounting pronouncements or tax guidelines, particularly with respect to RICs and business development companies;

​

•

loss of our qualification as a RIC or business development company;

​

•

changes in market interest rates and decline in the prices of debt;

​

•

changes in earnings or variations in operating results;

​

•

changes in the value of our portfolio investments;

​

•

changes in accounting guidelines governing valuation of our investments;

​

•

any shortfall in revenue or net income or any increase in losses from levels expected by investors or securities analysts;

​

•

departure of GC Advisors’ or any of its affiliates’ key personnel;

​

•

operating performance of companies comparable to us;

​

•

general economic trends and other external factors; and

​

•

loss of a major funding source.

​

If we issue preferred stock, debt securities or convertible debt securities, the net asset value and market value of our common stock may become more volatile.

We cannot assure you that the issuance of preferred stock and/or debt securities would result in a higher yield or return to the holders of our common stock. The issuance of preferred stock, debt securities or convertible debt would likely cause the net asset value and market value of our common stock to become more volatile. If the dividend rate on the preferred stock, or the interest rate on the debt securities, were to approach the net rate of return on our investment portfolio, the benefit of leverage to the holders of our common stock would be reduced. If the dividend rate on the preferred stock, or the interest rate on the debt

8

securities, were to exceed the net rate of return on our portfolio, the use of leverage would result in a lower rate of return to the holders of common stock than if we had not issued the preferred stock or debt securities. Any decline in the net asset value of our investment would be borne entirely by the holders of our common stock. Therefore, if the market value of our portfolio were to decline, the leverage would result in a greater decrease in net asset value to the holders of our common stock than if we were not leveraged through the issuance of preferred stock. This decline in net asset value would also tend to cause a greater decline in the market price for our common stock.

There is also a risk that, in the event of a sharp decline in the value of our net assets, we would be in danger of failing to maintain required asset coverage ratios which may be required by the preferred stock, debt securities, convertible debt or units or of a downgrade in the ratings of the preferred stock, debt securities, convertible debt or units or our current investment income might not be sufficient to meet the dividend requirements on the preferred stock or the interest payments on the debt securities. In order to counteract such an event, we might need to liquidate investments in order to fund redemption of some or all of the preferred stock, debt securities or convertible debt. In addition, we would pay (and the holders of our common stock would bear) all costs and expenses relating to the issuance and ongoing maintenance of the preferred stock, debt securities, convertible debt or any combination of these securities. Holders of preferred stock, debt securities or convertible debt may have different interests than holders of common stock and may at times have disproportionate influence over our affairs.

We are a holding company and depend on payments from our subsidiaries in order to make payments on any debt securities that we may issue as well as to pay dividends on our common stock. Any debt securities that we issue will be structurally subordinated to the obligations of our subsidiaries.

We are a holding company and fund a majority of our investments through wholly-owned subsidiaries, and a majority of the assets that we hold directly are the equity interests in such subsidiaries, including any subordinated notes issued as part of our debt securitization transactions, which notes represent the residual claimant on distributions by the applicable securitization subsidiary. We depend upon the cash flow from our subsidiaries and the receipt of funds from them in the form of payments on any subordinated notes, dividends, and other distributions, any of which may be subject to restriction or limitations based on the organizational documents of the subsidiaries and the agreements governing the debt of any such subsidiary. In addition, because we are a holding company, any debt securities that we issue will be structurally subordinated to the obligations of our subsidiaries. In the event that one of our subsidiaries becomes insolvent, liquidates, reorganizes, dissolves or otherwise winds up, its assets will be used first to satisfy the claims of its creditors. Consequently, any claim by us or our creditors, including holders of any debt securities that we may issue, against any subsidiary will be structurally subordinated to all of the claims of the creditors of such subsidiary. We cannot assure security holders that they will receive any payments required to be made under the terms of any debt securities that we may issue, dividends or other distributions.

Holders of any preferred stock that we may issue will have the right to elect members of the board of directors and have class voting rights on certain matters.

The 1940 Act requires that holders of shares of preferred stock must be entitled as a class to elect two directors at all times and to elect a majority of the directors if dividends on such preferred stock are in arrears by two years or more, until such arrearage is eliminated. In addition, certain matters under the 1940 Act require the separate vote of the holders of any issued and outstanding preferred stock, including changes in fundamental investment restrictions and conversion to open-end status and, accordingly, preferred stockholders could veto any such changes. Restrictions imposed on the declarations and payment of dividends or other distributions to the holders of our common stock and preferred stock, both by the 1940 Act and by requirements imposed by rating agencies, might impair our ability to maintain our qualification as a RIC for U.S. federal income tax purposes.

Your interest in us may be diluted if you do not fully exercise your subscription rights in any rights offering. In addition, if the subscription price is less than our net asset value per share, then you will experience an immediate dilution of the aggregate net asset value of your shares.

In the event we issue subscription rights, stockholders who do not fully exercise their subscription rights should expect that they will, at the completion of a rights offering pursuant to this prospectus, own a

9

smaller proportional interest in us than would otherwise be the case if they fully exercised their rights. We cannot state precisely the amount of any such dilution in share ownership because we do not know at this time what proportion of the shares will be purchased as a result of such rights offering.

In addition, if the subscription price is less than the net asset value per share of our common stock, then our stockholders would experience an immediate dilution of the aggregate net asset value of their shares as a result of the offering. The amount of any decrease in net asset value is not predictable because it is not known at this time what the subscription price and net asset value per share will be on the expiration date of a rights offering or what proportion of the shares will be purchased as a result of such rights offering. Such dilution could be substantial.

These dilutive effects may be exacerbated if we were to conduct multiple subscription rights offerings, particularly if such offerings were to occur over a short period of time. In addition, subscription rights offerings and the prospect of future subscription rights offerings may create downward pressure on the secondary market price of our common stock due to the potential for the issuance of shares at a price below our net asset value, without a corresponding change to our net asset value.

The trading market or market value of our publicly issued debt securities may fluctuate.

Our publicly issued debt securities may or may not have an established trading market. We cannot assure you that a trading market for our publicly issued debt securities will ever develop or be maintained if developed. In addition to our creditworthiness, many factors may materially adversely affect the trading market for, and market value of, our publicly issued debt securities. These factors include, but are not limited to, the following:

•

the time remaining to the maturity of these debt securities;

​

•

the outstanding principal amount of debt securities with terms identical to these debt securities;

​

•

the ratings assigned by national statistical ratings agencies;

​

•

the general economic environment;

​

•

the supply of debt securities trading in the secondary market, if any;

​

•

the redemption or repayment features, if any, of these debt securities;

​

•

the level, direction and volatility of market interest rates generally; and

​

•

market rates of interest higher or lower than rates borne by the debt securities.

​

You should also be aware that there may be a limited number of buyers when you decide to sell your debt securities. This too may materially adversely affect the market value of the debt securities or the trading market for the debt securities.

Terms relating to redemption may materially adversely affect your return on any debt securities that we may issue.

If your debt securities are redeemable at our option, we may choose to redeem your debt securities at times when prevailing interest rates are lower than the interest rate paid on your debt securities. In addition, if your debt securities are subject to mandatory redemption, we may be required to redeem your debt securities also at times when prevailing interest rates are lower than the interest rate paid on your debt securities. In this circumstance, you may not be able to reinvest the redemption proceeds in a comparable security at an effective interest rate as high as your debt securities being redeemed.

Our credit ratings may not reflect all risks of an investment in our debt securities.

Our credit ratings are an assessment by third parties of our ability to pay our obligations. Consequently, real or anticipated changes in our credit ratings will generally affect the market value of our debt securities. Our credit ratings, however, may not reflect the potential impact of risks related to market conditions generally or other factors discussed above on the market value of or trading market for the publicly issued debt securities.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus, including the documents that we incorporate by reference herein, contains, and any applicable prospectus supplement or free writing prospectus, including the documents we incorporate by reference therein, may contain forward-looking statements, which relate to future events or our future performance or financial condition. All statements other than statements of historical facts, including statements regarding our future events or future performance or financial condition, are forward-looking statements. The forward-looking statements contained or incorporated by reference in this prospectus and any applicable prospectus supplement or free writing prospectus may involve risks and uncertainties, including statements as to:

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our future operating results;

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our business prospects and the prospects of our portfolio companies;

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the effect of investments that we expect to make and the competition for those investments;

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our contractual arrangements and relationships with third parties;

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actual and potential conflicts of interest with GC Advisors and other affiliates of Golub Capital;

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the dependence of our future success on the general economy and its effect on the industries in which we invest;

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the ability of our portfolio companies to achieve their objectives;

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the use of borrowed money to finance a portion of our investments;

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the adequacy of our financing sources and working capital;

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the timing of cash flows, if any, from the operations of our portfolio companies;

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general economic and political trends and other external factors;

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the ability of GC Advisors to locate suitable investments for us and to monitor and administer our investments;

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the ability of GC Advisors or its affiliates to attract and retain highly talented professionals;

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our ability to qualify and maintain our qualification as a RIC and as a business development company;

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general price and volume fluctuations in the stock markets;

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the impact on our business of Dodd-Frank and the rules and regulations issued thereunder and any actions toward repeal thereof;

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the effect of changes to tax legislation and our tax position; and

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the transactions contemplated by the Agreement and Plan of Merger, dated as of November 27, 2018, that we entered into with Golub Capital Investment Corporation, GC Advisors, Fifth Ave Subsidiary Inc., our wholly-owned subsidiary, and, for certain limited purposes, the Administrator, or the Merger, the likelihood the Merger is completed and the anticipated timing of its completion.

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Such forward-looking statements may include statements preceded by, followed by or that otherwise include the words “may,” “might,” “will,” “intend,” “should,” “could,” “can,” “would,” “expect,” “believe,” “estimate,” “anticipate,” “predict,” “potential,” “plan” or similar words. The forward-looking statements contained in this prospectus and any applicable prospectus supplement or free writing prospectus involve risks and uncertainties. Our actual results could differ materially from those implied or expressed in the forward-looking statements for any reason, including the factors set forth as “Risk Factors” in our most recent Annual Report on Form 10-K and elsewhere contained or incorporated by reference in this prospectus and any applicable prospectus supplement or free writing prospectus.

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Discussions containing forward-looking statements may be found in the sections titled “Business,” “Risk Factors,” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” included in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC. We discuss in greater detail, and incorporate by reference into this prospectus in their entirety, many of these risks and uncertainties in the sections titled “Risk Factors” in the applicable prospectus supplement, in the free writing prospectus we may authorize for use in connection with a specific offering, and in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC.

We base the forward-looking statements included in this prospectus, any prospectus supplement, free writing prospectus and documents incorporated by reference into this prospectus on information available to us on the applicable date of the relevant document. Actual results could differ materially from those anticipated in our forward-looking statements and future results could differ materially from historical performance. You are advised to consult any additional disclosures that we may make directly to you or through reports that we have filed or in the future may file with the SEC, including annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K. This prospectus, any prospectus supplement, free writing prospectus and documents incorporated by reference into this prospectus contains or may contain statistics and other data that have been obtained from or compiled from information made available by third-party service providers. We have not independently verified such statistics or data.

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USE OF PROCEEDS

Unless otherwise specified in a prospectus supplement or a free writing prospectus we have authorized for use in connection with a specific offering, we intend to use all or substantially all of the net proceeds from the sale of our securities to invest in portfolio companies in accordance with our investment objective and strategies and for general corporate purposes. We expect that our new investments will consist primarily of one stop and other senior secured loans. We will also pay operating expenses, including management and administrative fees, and may pay other expenses such as due diligence expenses relating to potential new investments, from the net proceeds of any offering of our securities. We may also use a portion of the net proceeds from the sale of our securities to repay amounts outstanding under our Revolving Credit Facilities.

We anticipate that we will use substantially all of the net proceeds of an offering for the above purposes within approximately six months after the completion of any offering of our securities, depending on the availability of appropriate investment opportunities consistent with our investment objective and market conditions. We cannot assure you that we will achieve our targeted investment pace.

Until appropriate investment opportunities can be found, we may also invest the net proceeds of any offering of our securities primarily in cash, cash equivalents, U.S. government securities and high-quality debt investments that mature in one year or less from the date of investment. These temporary investments may have lower yields than our targeted investment types and, accordingly, may result in lower distributions, if any, during such period. Our ability to achieve our investment objective may be limited to the extent that the net proceeds from an offering, pending full investment, are held in lower yielding interest-bearing deposits or other short-term instruments. See “Business — Regulation — Temporary Investments” included in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC, for additional information about temporary investments we may make while waiting to make longer-term investments in pursuit of our investment objective.

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PRICE RANGE OF COMMON STOCK

Our common stock is currently traded on The Nasdaq Global Select Market under the symbol “GBDC”. The following table sets forth: (i) the net asset value per share of our common stock as of the applicable period end, (ii) the range of high and low closing sales prices of our common stock as reported on the Nasdaq during the applicable period, (iii) the closing high and low sales prices as a premium (discount) to net asset value during the relevant period, and (iv) the dividends and distributions per share of our common stock declared during the applicable period.

(1)

NAV per share is determined as of the last day in the relevant quarter and therefore may not reflect the NAV per share on the date of the high and low closing sales prices. The NAVs shown are based on outstanding shares at the end of each period.

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(2)

Calculated as of the respective high or low closing sales price divided by the quarter-end NAV.

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(3)

On May 7, 2019, our Board declared a quarterly distribution of  $0.32 per share payable on June 28, 2019 to holders of record of common stock as of June 7, 2019.

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(4)

Includes a special distribution of  $0.12 per share.

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(5)

Includes a special distribution of  $0.08 per share.

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(6)

Includes a special distribution of  $0.25 per share.

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The last reported price for our common stock on June 25, 2019 was $17.63 per share. As of June 21, 2019, we had 439 stockholders of record.

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SENIOR SECURITIES

Information about our senior securities is shown as of the dates indicated in the below table which is derived from our consolidated financial statements and related notes. This information about our senior securities should be read in conjunction with our audited and unaudited consolidated financial statements and related notes thereto and the section titled “Management’s Discussion and Analysis of Financial Condition and Results of Operations” included in our most recent Annual Report on Form 10-K and Quarterly Report on Form 10-Q, as well as any amendments reflected in subsequent filings with the SEC.

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