Filed pursuant to Rule 424(b)(2)

File No. 333-286240

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SUBJECT TO COMPLETION DATED SEPTEMBER 19, 2025

PRELIMINARY PROSPECTUS SUPPLEMENT

(To Prospectus dated March 28, 2025)

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GOLUB CAPITAL BDC, INC.

7.050% Notes due 2028

We are offering for sale $      million in aggregate principal amount of 7.050% notes due 2028, which we refer to as the Notes. The Notes will mature on December 5, 2028. We pay interest on the Notes semi-annually in arrears on June 5 and December 5 of each year. The Notes offered hereby are a further issuance of the 7.050% notes due 2028 that we issued on December 5, 2023 in the aggregate principal amount of $450.0 million (the “Existing Notes”). The Notes offered hereby will be treated as a single series with the Existing Notes under the indenture and will have the same terms as the Existing Notes (except the issue date, the offering price and the initial interest payment date). The Notes offered hereby will have the same CUSIP number and will be fungible and rank equally with the Existing Notes. Upon the issuance of the Notes offered hereby, the outstanding aggregate principal amount of our 7.050% notes due 2028 will be $      million. Unless the context otherwise requires, references herein to the “Notes” include the Notes offered hereby and the Existing Notes.

We may redeem the Notes in whole or in part at any time, or from time to time, at the applicable redemption price discussed under the caption “Description of Notes — Optional Redemption” in this prospectus supplement. In addition, holders of the Notes can require us to repurchase some or all of the Notes at a purchase price equal to 100% of their principal amount, plus accrued and unpaid interest to, but not including, the repurchase date upon the occurrence of a Change of Control Repurchase Event (as defined herein). The Notes will be issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof.

The Notes are our general unsecured obligations that rank senior in right of payment to all of our future indebtedness that is expressly subordinated in right of payment to the Notes, rank pari passu with all existing and future unsecured unsubordinated indebtedness issued by us (including the 2026 Notes, the 2027 Notes, the 2029 Notes and the Existing Notes, each as defined herein), rank effectively junior to all of our existing and future secured indebtedness (including unsecured indebtedness that we later secure) to the extent of the value of the assets securing such indebtedness, and rank structurally junior to all existing and future indebtedness (including trade payables) incurred by our subsidiaries, financing vehicles or similar facilities. As of September 18, 2025, we had approximately $4.9 billion of debt outstanding, $2.2 billion of which was unsecured senior indebtedness (represented by the 2026 Notes, the 2027 Notes, the 2029 Notes and the Existing Notes) that will rank equal to the Notes, approximately $1.4 billion of which was indebtedness secured by substantially all of the assets of our subsidiaries and that will be structurally senior to the Notes, and approximately $1.3 billion of which was indebtedness secured by substantially all of our assets and that will be effectively senior to the Notes. We do not presently expect to issue any subordinated debt.

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 (a loan that combines characteristics of traditional first lien senior secured loans and second lien or subordinated loans and that are often referred to by other middle-market lenders as unitranche loans) and other senior secured loans of U.S. middle-market companies. We 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 U.S. middle-market companies with over $80.0 billion in capital under management as of July 1, 2025.

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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, in “Risk Factors” beginning on page S-11 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 this 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 Notes. 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 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 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 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 often 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 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.

The underwriter has agreed to purchase the New Notes from us at a price of         % of par plus accrued and unpaid interest of $        in the aggregate from and including June 5, 2025 up to, but not including the date of delivery, which will result in $        of proceeds to us before expenses. On December 5, 2025, we will pay this pre-issuance accrued interest to the holders of the New Notes as of the applicable record date along with the interest accrued on the New Notes from the date of delivery to such interest payment date. The underwriter proposes to offer the New Notes for sale, from time to time, in one or more negotiated transactions, at varying prices plus accrued and unpaid interest from and including June 5, 2025 up to, but not including the date of delivery. These sales may occur at market prices prevailing at the time of sale or at prices negotiated by the underwriter.

THE NOTES ARE NOT DEPOSITS OR OTHER OBLIGATIONS OF A BANK AND ARE NOT INSURED BY THE FEDERAL DEPOSIT INSURANCE CORPORATION OR ANY OTHER GOVERNMENT AGENCY.

Delivery of the Notes offered hereby in book-entry form only through The Depository Trust Company will be made on or about          , 2025.

Sole Book Running Manager

RBC Capital Markets

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The date of this prospectus supplement is          , 2025.

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ABOUT THIS PROSPECTUS SUPPLEMENT

We have not, and the underwriter has not, authorized any other person to provide you with any information other than the information included or incorporated by reference in this prospectus supplement and the accompanying prospectus or in any free writing prospectus prepared by us or on behalf of us that relates to this offering. We and the underwriter take no responsibility for, and can provide no assurances as to the reliability of, any other information that others may give you. We are not, and the underwriter is not, making an offer to sell the Notes 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 the Notes 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, the Notes only in jurisdictions where offers are permitted.

We have filed with the SEC a registration statement on Form N-2 (File No. 333-286240) utilizing a shelf registration process relating to the securities described in this prospectus supplement. 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 the 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, the accompanying prospectus and the documents incorporated by reference herein and therein and “Available Information” included in this prospectus supplement before you make an investment decision.

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TABLE OF CONTENTS

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PROSPECTUS SUPPLEMENT

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PROSPECTUS

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PROSPECTUS SUPPLEMENT SUMMARY

This summary highlights information included elsewhere in this prospectus supplement and the accompanying prospectus or incorporated by reference in this prospectus supplement or the accompanying prospectus. It is not complete and may not contain all of the information that you should consider before making your investment decision. You should carefully read the entire prospectus supplement, the accompanying prospectus, and any related free writing prospectus, including the risks of investing in the Notes discussed in the section titled “Risk Factors” in this prospectus supplement and the accompanying prospectus and any related free writing prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus supplement.

Except as otherwise indicated, the terms:

Golub Capital BDC

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 1940 Act. In addition, for U.S. federal income tax purposes, we have elected to be treated as a regulated investment company, or RIC, under Subchapter M of the Internal Revenue Code of 1986, as amended, or the Code. We were formed in November 2009 to continue and expand the business of our predecessor, Golub Capital Master Funding LLC, which commenced operations in July 2007. We make investments primarily in one stop (a loan that combines characteristics of traditional first lien senior secured loans and second lien or subordinated loans and that are often referred to by other middle-market lenders as unitranche loans) and other senior secured loans of U.S. middle-market companies that are, in most cases, sponsored by private equity firms. In this prospectus supplement, the term “middle-market” generally refers to companies having earnings before interest, taxes, depreciation and amortization of less than $100.0 million annually. GC Advisors structures these one stop loans as senior secured loans, and we obtain security interests in the assets of the portfolio company that serve as collateral in support of the repayment of these loans. This collateral often takes the form of first-priority liens on the assets of the portfolio company. In many cases, we are the sole lender or we, together with our affiliates, are the sole lenders of one stop loans, which can afford us additional influence over the borrower in terms of monitoring and, if necessary, remediating any underperformance.

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 in industries we believe are resistant to recessions. We also selectively invest in second lien and subordinated (a loan that ranks senior only to a borrower’s equity securities and ranks junior to all of such borrower’s other indebtedness in priority of payment) loans of, and warrants and minority equity securities in, U.S. middle-market companies.

We intend to achieve our investment objective by (1) accessing the established loan origination channels developed by Golub Capital, a leading lender to U.S. middle-market companies with over $80.0 billion of capital under management as of July 1, 2025, (2) selecting investments within our core middle-market company focus, (3) partnering with experienced private equity firms, or sponsors, in many cases with whom Golub Capital has invested alongside in the past, (4) implementing the disciplined underwriting standards of Golub Capital and (5) drawing upon the aggregate experience and resources of Golub Capital.

We seek to create a portfolio that includes primarily one stop and other senior secured loans by primarily investing approximately $10.0 million to $80.0 million of capital, on average, in the securities of U.S. middle-market companies. We also selectively invest more than $80.0 million in some of our portfolio companies and generally expect that the size of our individual investments will vary proportionately with the size of our capital base.

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 often 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.

Our Adviser

Our investment activities are managed by our investment adviser, GC Advisors. GC Advisors is responsible for sourcing potential investments, conducting research and due diligence on prospective investments and equity sponsors, analyzing investment opportunities, structuring our investments and monitoring our investments and portfolio companies on an ongoing basis. GC Advisors was organized in September 2008 and is a registered investment adviser under the Investment Advisers Act of 1940, as amended. Under the Investment Advisory Agreement, we pay GC Advisors a base management fee and an incentive fee for its services. See “Item 1. Business — Management Agreements — Management Fee” included in our most recent Annual Report on Form 10-K, for a discussion of the base management fee and incentive fee, including the income and capital gains incentive fee, payable by us to GC Advisors and the incentive fee cap. Unlike most closed-end funds whose fees are based on assets net of leverage, our base management fee is based on our average-adjusted gross assets (including leverage but adjusted to exclude cash and cash equivalents so that investors do not pay the base management fee on such assets) and, therefore, GC Advisors benefits when we incur debt or use leverage. For purposes of the Investment Advisory Agreement, cash equivalents means U.S. government securities and commercial paper instruments maturing within 270 days of purchase.

Additionally, under the incentive fee structure, GC Advisors benefits when capital gains are recognized and, because it determines when a holding is sold, GC Advisors controls the timing of the recognition of capital gains. Our board of directors is charged with protecting our interests by monitoring how GC Advisors addresses these and other conflicts of interest associated with its management services and compensation.

While not expected to review or approve each borrowing, our independent directors, who are not “interested persons” as defined in Section 2(a)(19) of the 1940 Act, periodically review GC Advisors’ services and fees as well as its portfolio management decisions and portfolio performance. In connection with these reviews, our independent directors consider whether our fees and expenses (including those related to leverage) remain appropriate. See “Item 1. Business — Management Agreements — Board Approval of the Investment Advisory Agreement” included in our most recent Annual Report on Form 10-K.

GC Advisors is an affiliate of Golub Capital and pursuant to a staffing agreement, or the Staffing Agreement, Golub Capital LLC makes experienced investment professionals available to GC Advisors and provides access to the senior investment personnel of Golub Capital LLC and its affiliates. The Staffing Agreement provides GC Advisors with access to investment opportunities, which we refer to in the aggregate as deal flow, generated by Golub Capital LLC and its affiliates in the ordinary course of their businesses and commits the members of GC Advisors’ investment committee to serve in that capacity. As our investment adviser, GC Advisors is obligated to allocate investment opportunities among us and its other clients fairly and equitably over time in accordance with its allocation policy. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations — Related Party Transactions” in our most recent Annual Report on Form 10-K and Quarterly Report on Form 10-Q. However, there can be no assurance that such opportunities will be allocated to us fairly or equitably in the short-term or over time. GC Advisors seeks to capitalize on the significant deal origination, credit underwriting, due diligence, investment structuring, execution, portfolio management and monitoring experience of Golub Capital LLC’s investment professionals.

An affiliate of GC Advisors, the Administrator, provides the administrative services necessary for us to operate. See “Item 1. Business — Management Agreements — Administration Agreement” included in our most recent Annual Report on Form 10-K,for a discussion of the fees and expenses (subject to the review and approval of our independent directors) we are required to reimburse to the Administrator.

About Golub Capital

Golub Capital, founded in 1994, is a leading lender to middle-market companies, with a long track record of investing in senior secured, one stop, second lien and subordinated loans. As of July 1, 2025, Golub Capital had over $80.0 billion in capital under management. Since its inception, Golub Capital has closed deals with over 410 middle-market sponsors and repeat transactions with over 280 sponsors as of June 30, 2025.

Golub Capital’s middle-market lending group is managed by an eight-member senior management team consisting of Lawrence E. Golub, David B. Golub, Andrew H. Steuerman, Gregory W. Cashman, Spyro G. Alexopoulos, Marc C. Robinson, Robert G. Tuchscherer and Jason J. Van Dussen. As of June 30, 2025, Golub Capital had more than 230 investment professionals supported by more than 800 administrative and back office personnel that focus on operations, finance, legal and compliance, accounting and reporting, marketing, information technology and office management.

Corporate Information

Our principal executive offices are located at 200 Park Avenue, 25th Floor, New York, NY 10166, and our telephone number is (212) 750-6060. Our corporate website is located at www.golubcapitalbdc.com.

Information on our website is not incorporated into or a part of this prospectus supplement or the accompanying prospectus.

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SPECIFIC TERMS OF THE NOTES AND THE OFFERING

This prospectus supplement sets forth certain terms of the Notes that we are offering pursuant to this prospectus supplement and supplements the accompanying prospectus that is attached to this prospectus supplement. On December 5, 2023, we and U.S. Bank Trust Company, National Association (as successor in interest to U.S. Bank National Association), as trustee (“U.S. Bank” or the “trustee”) entered into the fourth supplemental indenture to the indenture, dated as of October 2, 2020, relating to the Existing Notes. We will issue the Notes offered hereby under the same base indenture and the fourth supplemental indenture, which we refer to collectively as the “indenture.” The Notes offered hereby will be a further issuance of, rank equally in right of payment with, and form a single series with the Existing Notes for all purposes under the indenture, including waivers, amendments, consents, redemptions and other offers to purchase and voting. We refer to the “Notes” and the “Existing Notes” separately within this prospectus supplement because only the Notes are being offered hereby, but any general discussion of the terms of the Notes also would apply to the Existing Notes because they are treated as the same under the indenture. This section outlines the specific legal and financial terms of the Notes. You should read this section together with the more general description of the Notes under the heading “Description of Notes” in this prospectus supplement and in the accompanying prospectus under the heading “Description of Our Debt Securities” before investing in the Notes. Capitalized terms used in this prospectus supplement and not otherwise defined shall have the meanings ascribed to them in the accompanying prospectus or in the indenture governing the Notes.

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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 Notes 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 reference into this prospectus supplement and the accompanying prospectus, and any related free writing prospectus, involve risks and uncertainties, including statements as to:

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 as a result of the factors set forth as “Risk Factors” in our most recent Annual Report on Form 10-K and elsewhere contained, or incorporated by reference into, this prospectus supplement and the accompanying prospectus.

We have based the forward-looking statements included in this prospectus supplement or the accompanying prospectus, including the 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 will 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 the 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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FINANCIAL HIGHLIGHTS

The financial highlights of Golub Capital BDC for the fiscal years ended September 30, 2024, 2023, 2022, 2021 and 2020 and as of the nine months ended June 30, 2025 and 2024 are as follows:

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The financial highlights of Golub Capital BDC for the fiscal years ended September 30, 2019, 2018, 2017, 2016, and 2015 are as follows:

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USE OF PROCEEDS

We estimate that the net proceeds we will receive from the sale of the $            aggregate principal amount of the Notes in this offering will be approximately $          after deducting the underwriting discounts and commissions of $          payable by us and estimated offering expenses of approximately $          payable by us.

We intend to use the net proceeds from this offering to repay a portion of the outstanding indebtedness under the JPM Credit Facility. However, we may re-borrow under the JPM Credit Facility or borrow under the Adviser Revolver for general corporate purposes, which may include investing in portfolio companies in accordance with our investment strategy and for general corporate purposes.

As of September 18, 2025, we had approximately $4.9 billion of debt outstanding, $2.2 billion of which was unsecured senior indebtedness (represented by the 2026 Notes, the 2027 Notes, the 2029 Notes, and the Existing Notes) that will rank equal to the Notes, approximately $1.4 billion of which was indebtedness secured by substantially all of the assets of our subsidiaries and that will be structurally senior to the Notes, and approximately $1.3 billion of which was indebtedness secured by substantially all of our assets and that will be effectively senior to the Notes.

The JPM Credit Facility matures on August 6, 2029, and borrowings under the JPM Credit Facility currently bear interest at the applicable base rate plus an applicable margin ranging between 1.525% to 1.775% for any Term Benchmark Loan or RFR Loan, subject to compliance with a borrowing base rate. The applicable base rate under the JPM Credit Facility is (i) one-month SOFR with respect to any advances denominated in U.S. dollars plus an adjustment of 0.10%, (ii) SONIA with respect to any advances denominated in U.K. pound sterling plus an adjustment of 0.0326%, (iii) one-month EURIBOR with respect to any advances denominated in Euros, (iv) CORRA with respect to any advances denominated in Canadian Dollars plus an adjustment of 0.029547%, (v) Australian Bank Bill Swap Bid Rate with respect to any advances denominated in Australian Dollars plus an adjustment of 0.20% and (vi) the relevant rate as defined in the JPM Credit Facility for borrowings in other currencies. As of September 18, 2025, there was $1.3 billion outstanding under the JPM Credit Facility.

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CAPITALIZATION

The following table sets forth our cash and capitalization as of June 30, 2025:

You should read this table together with “Use of Proceeds” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and our consolidated financial statements and related notes thereto included in our most recent Annual Report on Form 10-K and Quarterly Report on Form 10-Q.

Dollar amounts are presented in thousands, except share data.

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CERTAIN U.S. FEDERAL INCOME TAX CONSIDERATIONS

The following discussion is a general summary of certain U.S. federal income tax considerations applicable to purchasing, owning, and disposing of the Notes. This summary addresses only those holders who purchase Notes in this offering at the public offering price. Moreover, this summary does not purport to be a complete description of the income tax considerations applicable to such an investment and does not address any state, local or non-U.S. income or other tax considerations. The discussion is based upon the Code, the regulations promulgated thereunder by the U.S. Department of the Treasury, or the “Treasury Regulations,” and administrative and judicial interpretations, each as of the date of this prospectus supplement and all of which are subject to change, potentially with retroactive and adverse effect. Investors should consult their own tax advisors with respect to tax considerations that pertain to their investment in the Notes. We have not sought any ruling from the Internal Revenue Service, or IRS, with respect to the statements made and the conclusions reached in the following discussion, and there can be no assurance that the IRS will agree with such statements and conclusions.

This discussion deals only with Notes held as capital assets within the meaning of Section 1221 of the Code and does not cover all possible income tax considerations that may be applicable to beneficial owners of the Notes (referred to in this discussion as “holders”) with special circumstances, including, without limitation, the U.S. federal income tax consequences applicable to holders such as any government (or instrumentality or agency thereof), financial institutions, insurance companies, real estate investment trusts, controlled foreign corporations, passive foreign investment companies and regulated investment companies (and shareholders of such corporations), pension plans, trusts and estates, dealers in securities or currencies, traders in securities, certain former citizens and long term residents of the United States and certain former citizens or long-term residents of the United States, persons holding the Notes as a hedge against currency risks or as a position in a “straddle,” “hedge,” “constructive sale transaction” or “conversion transaction” (as those terms are defined under the Code), entities that are tax-exempt for U.S. federal income tax purposes, retirement plans, individual retirement accounts, tax-deferred accounts, persons subject to the alternative minimum tax, pass-through entities (including partnerships and other entities and arrangements classified as partnerships for U.S. federal income tax purposes) and beneficial owners of such pass-through entities, persons holding the Notes as intermediaries, agents or nominees, or persons whose functional currency (as defined in Section 985(b) of the Code) is not the U.S. dollar. This discussion also does not address the U.S. federal income tax consequences to beneficial owners of the Notes subject to the special tax accounting rules under Section 451(b) of the Code. This discussion is limited to persons purchasing the Notes for cash at original issue and at their original “issue price” within the meaning of Section 1273(b) of the Code (i.e., the initial price at which a substantial amount of the Notes are sold to the public for cash). This discussion does not address the effects of other U.S. federal tax laws (such as estate and gift tax laws) and any applicable state, local or non-U.S. tax laws (except where otherwise indicated). Investors considering purchasing the Notes should consult their own tax advisors concerning the application of the U.S. federal, state and local tax laws to their individual circumstances, as well as any consequences to such investors relating to purchasing, owning and disposing of the Notes under the laws of any non-U.S. taxing jurisdiction.

For purposes of this discussion, the term “U.S. holder” means a beneficial owner of a Note that is, for U.S. federal income tax purposes, (i) a citizen or individual resident of the United States, including an alien individual who is a lawful permanent resident of the United States or meets the “substantial presence” test under Section 7701(b)(3) of the Code, (ii) a corporation or other entity treated as a corporation, created or organized in or under the laws of the United States, any state thereof, or the District of Columbia, (iii) a trust (a) subject to the control of one or more United States persons (as defined under Section 7701(a)(30) the Code) and the primary supervision of a court in the United States, or (b) that has in force a valid election (under applicable Treasury Regulations) to be treated as a United States person, or (iv) an estate the income of which is subject to U.S. federal income taxation regardless of its source. The term “non-U.S. holder” means a beneficial owner of a Note that is neither a U.S. holder nor a partnership (including an entity or arrangement treated as a partnership for U.S. federal income tax purposes).

If a partnership (including an entity or arrangement treated as a partnership for U.S. federal income tax purposes) holds any Notes, the U.S. federal income tax treatment of a partner, member or owner of such entity generally will depend upon the status of such partner, member or owner, the activities of such entity and certain determinations made at the partner, member or owner level. Such entities holding Notes, and persons holding interests in such entities, should each consult their own tax advisors as to the consequences of investing in the Notes in their individual circumstances.

Qualified Reopening

We intend to treat, for U.S. federal income tax purposes, the issuance of the Notes offered hereby as a “qualified reopening” of the Existing Notes. Accordingly, we intend to treat the Notes offered hereby as having the same issue date and the same issue price as the Existing Notes. The remainder of this summary assumes this treatment.

Aggregate Accrued Interest

Interest on the Notes offered hereby will accrue from June 5, 2025, up to, but not including, the date of delivery and will be paid by the purchasers of the Notes offered hereby. On December 5, 2025, we will pay this Aggregate Accrued Interest to the holders of the Notes offered hereby as of the applicable record date along with interest accrued on the Notes offered hereby from the date of delivery to such interest payment date. We intend to take the position that a portion of the interest payment received on the first interest payment date following the issuance of the Notes that equals the Aggregate Accrued Interest as a return of the Aggregate Accrued Interest, rather than an amount payable on such Notes. If this position is respected a portion of the interest payment received by a holder on the first interest payment date following the issuance of the Notes will be treated as a non-taxable return of the Aggregate Accrued Interest paid by the holder, rather than as taxable interest, as if the holder purchased a debt instrument on the secondary market between interest payment dates and will, when received, reduce the holder’s basis in the Notes. Holders should consult their own tax advisors concerning the tax treatment of the Aggregate Accrued Interest on the Notes.

Taxation of U.S. Holders

Payments of Interest

The following discussion assumes that the Notes will be issued with no original issue discount or a de minimis amount of original issue discount for U.S. federal income tax purposes. Payments or accruals of “qualified stated interest” on a Note generally will be taxable to a U.S. holder as ordinary interest income at the time they are received (actually or constructively) or accrued, in accordance with the U.S. holder’s regular method of tax accounting. Qualified stated interest is stated interest (other than Aggregate Accrued Interest as described above) that is unconditionally payable in cash or in property (other than debt instruments of the issuer) at least annually at a single fixed rate.

Amortizable Bond Premium

If a U.S. holder purchases a Note in this offering for an amount (excluding any amount attributable to Aggregate Accrued Interest, described above) that exceeds the stated principal amount of the Note, such U.S. holder will be considered to have purchased the Note with amortizable bond premium equal to the amount of that excess. A U.S. holder generally may elect to amortize the premium using a constant yield method over the remaining term of the Note as an offset to interest when included in income in accordance with such U.S. holder’s regular method of tax accounting. Any amortized amount of the premium for a taxable year generally will be treated first as a reduction of interest on the Note includible in the U.S. holder’s gross income in such taxable year to the extent thereof, then as a deduction allowed in that taxable year to the extent of the U.S. holder’s prior interest inclusions on the Note, and finally as a carryforward allowable against the U.S. holder’s future interest inclusions on the Note. This election to amortize premium on a constant yield method will apply to all debt obligations (other than debt obligations the interest on which is excludable from gross income) held by such U.S. holder as of the beginning of, or acquired during or after, the first taxable year for which the election applies and may not be revoked without the consent of the IRS. If a U.S. holder makes the election to amortize bond premium with respect to a Note, such holder will be required to reduce its adjusted tax basis in such Note by the amount of the premium amortized. If a U.S. holder does not elect to amortize bond premium, that premium will decrease the gain or increase the loss such holder would otherwise recognize on the sale, exchange, redemption, retirement or other taxable disposition of the Note. Prospective investors should consult their own tax advisors regarding this election.

Market Discount

If the amount paid for a Note by a U.S. holder (excluding any amount attributable to Aggregate Accrued Interest) is less than the stated principal amount of the Note by more than a statutorily defined de minimis threshold, the U.S. holder generally would be considered to have “market discount” for U.S. federal income tax purposes equal to the amount of such difference. In the case of a Note acquired pursuant to this offering, a discount will be considered de minimis and thus will not give rise to market discount if it is less than 1/4 of 1% of the stated principal amount of the Note multiplied by the number of complete years to maturity. Under the market discount rules, market discount generally must be accrued over the term of the Note, and any partial payment of principal on a Note, or any gain on the sale, exchange, redemption, retirement or other disposition of a Note, generally must be treated as ordinary income to the extent of the accrued market discount (unless the holder makes an election to include the market discount in income as it accrues). In addition, a holder of a Note with market discount may be required to defer, until the maturity of the Note or its earlier disposition in a taxable transaction, the deduction of all or a portion of the interest expense on any indebtedness incurred or continued to purchase or carry the Note. Prospective purchasers of the Notes are urged to consult their tax advisors with respect to the rules relating to market discount and the application of those rules to their particular circumstances.

Sale or Other Taxable Disposition of Notes

Upon the sale, exchange, redemption, retirement or other taxable disposition of a Note, a U.S. holder generally will recognize capital gain or loss equal to the difference between the amount realized on the sale, exchange, redemption, retirement or other taxable disposition (excluding amounts representing accrued and unpaid interest (including any Aggregate Accrued Interest, as described above), which are treated as ordinary income to the extent not previously included in income) and the U.S. holder’s adjusted tax basis in the Note. A U.S. holder’s adjusted tax basis in a Note generally will equal the U.S. holder’s initial investment in the Note (excluding any amount attributable to the Aggregate Accrued Interest described above) less any principal payments received by the U.S. holder, reduced by the amount of any bond premium previously amortized by the U.S. holder with respect to the Note as well as any cash payments on the Note other than qualified stated interest or increased by any market discount previously included in the U.S. holder’s income with respect to a Note. Capital gain or loss generally will be long-term capital gain or loss if the U.S. holder’s holding period in the Note was more than one year. Long-term capital gains generally are taxed at reduced rates for individuals and certain other non-corporate U.S. holders, and the deductibility of capital losses is subject to limitations under the Code.

Information Reporting and Backup Withholding

A U.S. holder may be subject to information reporting and backup withholding when such U.S. holder receives interest payments on the Notes held or upon the proceeds received upon the sale or other disposition of such Notes (including a redemption or retirement of the Notes). Certain U.S. holders, such as corporations, generally are not subject to information reporting or backup withholding. A U.S. holder will be subject to backup withholding if such U.S. holder is not otherwise exempt and:

U.S. holders should consult their tax advisors regarding their qualification for an exemption from backup withholding and the procedures for obtaining such an exemption, if applicable. Backup withholding is not an additional tax, and taxpayers may use amounts withheld as a credit against their U.S. federal income tax liability or may claim a refund if they timely provide certain information to the IRS.

Unearned Income Medicare Contribution

A surtax of 3.8% will be imposed on certain “net investment income” (or “undistributed net investment income”, in the case of estates and trusts) received by individuals with modified adjusted gross incomes in excess of $200,000 ($250,000 in the case of married individuals filing jointly and $125,000 in the case of married individuals filing a separate return) and certain estates and trusts. “Net investment income” as defined for U.S. federal Medicare contribution purposes generally includes interest payments and gain recognized from the sale or other disposition of the Notes. Tax-exempt trusts, which are not subject to income taxes generally, and foreign individuals will not be subject to this tax. U.S. holders should consult their own tax advisors regarding the effect, if any, of this tax on their ownership and disposition of the Notes.

Taxation of Non-U.S. Holders

Payments of Interest

Generally, interest income paid to a non-U.S. holder that is not effectively connected with the non-U.S. holder’s conduct of a trade or business in the United States (a “U.S. trade or business”) is subject to withholding tax at a rate of 30% (or, if applicable, a lower treaty rate). Nevertheless, a non-U.S. holder generally will not be subject to U.S. federal income or withholding taxes on payments of interest on a Note, provided that (i) the non-U.S. holder is not a bank receiving interest described in Section 881(c)(3)(A) of the Code, (ii) does not own (actually or constructively) 10 percent or more of the total combined voting power of all classes of our stock, (iii) is not a controlled foreign corporation for U.S. federal income tax purposes that is related, directly or indirectly, to us through sufficient stock ownership and (iv) the non-U.S. holder provides, prior to payment, a statement on an IRS

Form W-8BEN or IRS Form W-8BEN-E (or other applicable form) signed under penalties of perjury that includes the non-U.S. holder’s name and address and certifies that it is not a United States person in compliance with applicable requirements. If a non-U.S. holder holds a Note through an intermediary, agent or nominee, such intermediary, agent or nominee must also provide a valid intermediary withholding certificate that complies with the applicable Treasury Regulations.

Even if the above conditions are not met, a non-U.S. holder generally will be entitled to a reduction in or an exemption from U.S. federal withholding tax on interest if the non-U.S. holder provides us or our paying agent with a properly executed IRS Form W-8BEN or IRS Form W-8BEN-E, as applicable, or a suitable substitute form (or other applicable certificate) claiming an exemption from or reduction of the withholding tax under the benefit of an income tax treaty between the United States and the non-U.S. holder’s country of residence. A non-U.S. holder is required to inform the recipient of any change in the information on such statement within 30 days of such change. Special certification rules apply to non-U.S. holders that are pass-through entities rather than corporations or individuals.

If interest paid to a non-U.S. holder is effectively connected with the non-U.S. holder’s conduct of a U.S. trade or business (and if required by an applicable income tax treaty, is attributable to a permanent establishment maintained by the non-U.S. holder in the United States), then, the non-U.S. holder will be exempt from U.S. federal withholding tax, so long as the non-U.S. holder has provided an IRS Form W-8ECI or substantially similar substitute form stating that the interest that the non-U.S. holder receives on the Notes is effectively connected with the non-U.S. holder’s U.S. trade or business. In such a case, a non-U.S. holder will be subject to tax on the interest it receives on a net income basis in the same manner as if such non-U.S. holder were a U.S. holder. In addition, if the non-U.S. holder is a foreign corporation, such interest may be subject to a branch profits tax at a rate of 30% or lower applicable treaty rate.

Sale or Other Taxable Disposition of Notes

Generally, subject to the discussions of backup withholding and FATCA (defined below) a non-U.S. holder will not be subject to U.S. federal income or withholding taxes on any amount that constitutes capital gain upon the sale, exchange, redemption, retirement or other taxable disposition of a Note, unless the gain is effectively connected with the conduct of a U.S. trade or business by the non-U.S. holder (and, if an income tax treaty applies, is attributable to a United States “permanent establishment” maintained by the non-U.S. holder). However, if an individual non-U.S. holder is present in the United States for 183 or more days, using the counting methodology described in the applicable Treasury Regulations, during the taxable year in which the sale, exchange, redemption, retirement or other taxable disposition of a Note occurs and is not otherwise treated as a U.S. holder under the substantial presence test under section 7701(b) of the Code, and certain other conditions exist, such non-U.S. holder will be subject to U.S. federal income tax, imposed at a 30 percent rate, on any resulting gain (except to the extent otherwise provided by an applicable income tax treaty), which may be offset by certain U.S. losses. Non-U.S. holders should consult their own tax advisors with regard to whether taxes will be imposed on capital gain in their individual circumstances.

Information Reporting and Backup Withholding

The amount of interest that we pay to any documented non-U.S. holder on the Notes will be reported to the non-U.S. holder and to the IRS annually on an IRS Form 1042-S, regardless of whether any tax was actually withheld. Copies of these information returns may also be made available under the provisions of a specific income tax treaty or agreement to the tax authorities of the country in which the non-U.S. holder resides. However, a non-U.S. holder generally will not be subject to backup withholding and certain other information reporting with respect to payments that we make to the non-U.S. holder, provided that we do not have actual knowledge or reason to know that such non-U.S. holder is a “United States person,” within the meaning of the Code, and the non-U.S. holder has given us the statement described above under “non-U.S. holders  — Payments of Interest.”

A non-U.S. holder generally will be entitled to credit any amounts withheld under the backup withholding rules against the non-U.S. holder’s U.S. federal income tax liability or may claim a refund provided that the required information is furnished to the IRS in a timely manner.

Non-U.S. holders are urged to consult their tax advisors regarding the application of information reporting and backup withholding in their particular situations, the availability of an exemption therefrom, and the procedures for obtaining such an exemption, if available.

Foreign Account Tax Compliance Act

Legislation commonly referred to as the “Foreign Account Tax Compliance Act,” or “FATCA,” generally imposes a 30% withholding tax on payments of certain types of income to foreign financial institutions, or FFIs, unless such FFIs either (i) enter into an agreement with the U.S. Treasury to report certain required information with respect to accounts held by certain specified U.S. persons (or held by foreign entities that have certain specified U.S. persons as substantial owners) or (ii) reside in a jurisdiction that has entered into an intergovernmental agreement, or IGA, with the United States to collect and share such information and are in compliance with the terms of such IGA and any enabling legislation or regulations. The types of income subject to the tax include U.S. source interest and dividends. While the Code would also require withholding on payments of the gross proceeds from the sale of any property that could produce U.S. source interest or dividends, the U.S. Treasury Department has indicated its intent to eliminate this requirement in subsequent proposed regulations, which state that taxpayers may rely on the proposed regulations until final regulations are issued. The information required to be reported includes the identity and taxpayer identification number of each account holder that is a specified U.S. person and certain financial information with respect to the holder’s account. In addition, subject to certain exceptions, this legislation also imposes a 30% withholding on certain payments to certain foreign entities that are not FFIs unless the foreign entity certifies that it does not have a greater than 10% owner that is a specified U.S. person or provides the withholding agent with identifying information on each greater than 10% owner that is a specified U.S. person. Depending on the status of a beneficial owner and the status of the intermediaries through which they hold their Notes, beneficial owners could be subject to this 30% withholding tax with respect to interest paid on the Notes and potentially on proceeds from the sale of the Notes to the extent treated as interest for U.S. federal income tax purposes. Under certain circumstances, a beneficial owner might be eligible for refunds or credits of such taxes.

THE PRECEDING DISCUSSION IS NOT INTENDED TO BE A COMPLETE DISCUSSION OF ALL THE APPLICABLE TAX CONSEQUENCES TO A HOLDER OF PURCHASING, OWNING OR DISPOSING OF THE NOTES, NOR IS IT INTENDED TO CONSTITUTE TAX ADVICE. PROSPECTIVE INVESTORS SHOULD CONSULT THEIR OWN TAX ADVISORS WITH RESPECT TO THE PARTICULAR TAX CONSEQUENCES OF AN INVESTMENT IN THE NOTES IN THEIR INDIVIDUAL CIRCUMSTANCES.

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UNDERWRITING

RBC Capital Markets, LLC is acting as the sole underwriter for this offering. Subject to the terms and conditions set forth in an underwriting agreement by and among us, GC Advisors, the Administrator and the underwriter, we have agreed to sell to the underwriter, and the underwriter has agreed to purchase from us, the aggregate principal amount of Notes set forth opposite its name below.

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Subject to the terms and conditions set forth in the underwriting agreement, the underwriter has agreed to purchase all of the Notes sold under the underwriting agreement if any of these Notes are purchased.

We, GC Advisors and the Administrator have agreed to indemnify the underwriter against certain liabilities, including liabilities under the Securities Act of 1933, as amended, or the Securities Act, or to contribute to payments the underwriter may be required to make in respect of those liabilities.

The underwriter is offering the Notes, subject to prior sale, when, as and if issued to and accepted by it, subject to approval of legal matters by its counsel, including the validity of the Notes, and other conditions contained in the underwriting agreement, such as the receipt by the underwriter of officers’ certificates and legal opinions. The underwriter reserves the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

Commissions and Discounts

The underwriter has agreed to purchase the New Notes from us at a price of          % of par plus accrued and unpaid interest of $          in the aggregate from and including June 5, 2025 up to, but not including the date of delivery, which will result in $          of proceeds to us before expenses. On December 5, 2025, we will pay this pre-issuance accrued interest to the holders of the New Notes as of the applicable record date along with the interest accrued on the New Notes from the date of delivery to such interest payment date. The underwriter proposes to offer the New Notes for sale, from time to time, in one or more negotiated transactions, at varying prices plus accrued and unpaid interest from and including June 5, 2025 up to, but not including the date of delivery. These sales may occur at market prices prevailing at the time of sale or at prices negotiated by the underwriter.

The expenses of the offering, not including the underwriting discount, are estimated at approximately $          and are payable by us. We will pay the fees and expenses (including reasonable legal fees and disbursements) incident to any required review by FINRA of the terms of the sale of the Notes in this offering in an amount not to exceed $10,000.

No Sales of Similar Securities

Until the settlement date of this offering, we, GC Advisors and the Administrator have agreed with the underwriter, subject to certain exceptions, not to offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend or otherwise transfer or dispose of any debt securities issued or guaranteed by us or any securities convertible into or exercisable or exchangeable for debt securities issued or guaranteed by us, whether now owned or hereafter acquired.

In addition, until the settlement date of this offering, we have agreed with the underwriter not to file or cause the filing of any registration statement under the Securities Act with respect to any debt securities issued or guaranteed by us or any securities convertible into or exercisable or exchangeable for debt securities issued or guaranteed by us.

Listing

While a trading market developed after issuing the Existing Notes, we cannot assure you that an active liquid market for the Notes will be maintained. The Notes will not be listed on any securities exchange or quoted on any automated dealer quotation system. Although the underwriter has informed us that it intends to continue to make a market in the Notes, as permitted by applicable laws and regulations, it is not obligated to do so, and may discontinue any such market-making activities at any time without notice.

Accordingly, no assurance can be given as to the liquidity or maintenance of a public trading market for the Notes. If an active public trading market for the Notes is not maintained, the market price and liquidity of the Notes may be adversely affected.

Price Stabilization and Short Positions

In connection with the offering, the underwriter may purchase and sell the Notes in the open market. These transactions may include short sales and purchases on the open market to cover positions created by short sales. Short sales involve the sale by the underwriter of a greater principal amount of Notes than they are required to purchase in the offering. The underwriter must close out any short position by purchasing Notes in the open market. A short position is more likely to be created if the underwriter is concerned that there may be downward pressure on the price of the Notes in the open market after pricing that could adversely affect investors who purchase in the offering.

Similar to other purchase transactions, the underwriter’s purchases to cover the syndicate short sales may have the effect of raising or maintaining the market price of the Notes or preventing or retarding a decline in the market price of the Notes. As a result, the price of the Notes may be higher than the price that might otherwise exist in the open market.

Any of these activities may cause the price of the Notes to be higher than the price that otherwise would exist in the open market in the absence of such transactions. These transactions may be affected in the over-the-counter market or otherwise and, if commenced, may be discontinued at any time without any notice relating thereto.

Neither we nor the underwriter make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of the Notes. In addition, neither we nor the underwriter make any representation that the representative will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

Alternative Settlement Cycle

We expect that delivery of the Notes offered hereby will be made against payment therefor on or about          , 2025, which will be the fifth business day following the date of the pricing of the Notes offered hereby (such settlement being herein referred to as “T+5”). Under Rule 15c6-1 promulgated under the Exchange Act, trades in the secondary market generally are required to settle in one business day, unless the parties to any such trade expressly agree otherwise. Accordingly, purchasers who wish to trade the Notes offered hereby prior to the first business day before the date of delivery hereunder will be required, by virtue of the fact that the Notes offered hereby initially will settle in T+5 business days, to specify an alternate settlement arrangement at the time of any such trade to prevent a failed settlement.

Electronic Offer, Sale and Distribution of Notes

The underwriter may make prospectuses available in electronic (PDF) format. A prospectus in electronic (PDF) format may be made available on a web site maintained by the underwriter, and the underwriter may distribute such prospectuses electronically.

Additional Relationships

The underwriter and its affiliates are full service financial institutions engaged in various activities, which may include sales and trading, commercial and investment banking, advisory, investment management, investment research, principal investment, hedging, market making, brokerage and other financial and non-financial activities and services. The underwriter and its affiliates have, from time to time, performed, and may in the future perform, various financial advisory and investment banking services for us and our affiliates, for which they received or will receive customary fees and expenses, including acting as underwriters for our and our affiliates’ securities offerings.

In the ordinary course of their various business activities, the underwriter and its affiliates, officers, directors and employees may purchase, sell or hold a broad array of investments and actively trade securities, derivatives, loans, commodities, currencies, credit default swaps and other financial instruments for their own account and for the accounts of their customers, and such investment and trading activities may involve or relate to assets, securities and/or instruments of ours and our affiliates (directly, as collateral securing other obligations or otherwise) and/or persons and entities with relationships with us and our affiliates. If the underwriter or its affiliates have a lending relationship with us, the underwriter or its affiliates routinely hedge, or may hedge, their credit exposure to us consistent with their customary risk management policies. Typically, the underwriter and its affiliates would hedge such exposure by

entering into transactions which consist of either the purchase of credit default swaps or the creation of short positions in our securities, including potentially the Notes offered hereby. Any such credit default swaps or short positions could adversely affect future trading prices of the Notes offered hereby. The underwriter and its affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of such assets, securities or instruments and may at any time hold, or recommend to clients that they should acquire, long and/or short positions in such assets, securities and instruments.

Affiliates of the underwriter serve as lenders under the JPM Credit Facility. The underwriter and its affiliates were underwriters in connection with our initial public offering and our subsequent common stock and notes offerings, for which they received customary fees.

The proceeds of this offering will be used to repay a portion of the outstanding indebtedness under the JPM Credit Facility. Accordingly, affiliates of the underwriter may receive more than 5% of the proceeds of this offering to the extent such proceeds are used to repay outstanding indebtedness under the JPM Credit Facility.

After the date of this prospectus supplement, the underwriter and its affiliates may from time to time obtain information regarding specific portfolio companies or us that may not be available to the general public. Any such information is obtained by the underwriter and its affiliates in the ordinary course of their business and not in connection with the offering of the Notes. In addition, after the offering period for the sale of the Notes, the underwriter or its affiliates may develop analyses or opinions related to Golub Capital LLC or our portfolio companies and buy or sell interests in one or more of our portfolio companies on behalf of their proprietary or client accounts and may engage in competitive activities. There is no obligation on behalf of these parties to disclose their respective analyses, opinions or purchase and sale activities regarding any portfolio company or regarding Golub Capital LLC to our investors.

The principal business address of RBC Capital Markets, LLC is Brookfield Place, 200 Vesey Street, 8^th Floor, New York, New York 10281.

Notice to Prospective Investors in Canada

The Notes may be sold only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument 45-106 Prospectus Exemptions or subsection 73.3(1) of the Securities Act (Ontario), and are permitted clients, as defined in National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the Notes must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus supplement (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 (or, in the case of securities issued or guaranteed by the government of a non- Canadian jurisdiction, section 3A.4) of National Instrument 33-105 Underwriting Conflicts (NI 33-105), the underwriter is not required to comply with the disclosure requirements of NI 33-105 regarding underwriter conflicts of interest in connection with this offering.

Notice to Prospective Investors in the European Economic Area

The Notes may not be offered, sold or otherwise made available to any retail investor in the European Economic Area, or EEA. For these purposes:

Consequently, no key information document required by Regulation (EU) No 1286/2014 (as amended, or the PRIIPs Regulation) for offering or selling the Notes or otherwise making them available to retail investors in the EEA has been prepared and therefore offering or selling the Notes or otherwise making them available to any retail investor in the EEA may be unlawful under the PRIIPs Regulation. This prospectus supplement and the accompanying prospectus have been prepared on the basis that any offer of Notes in any member state of the EEA will be made pursuant to an exemption under the Prospectus Regulation from the requirement to publish a prospectus for offers of the Notes. This prospectus supplement and the accompanying prospectus are not a prospectus for the purposes of the Prospectus Regulation.

Notice to Prospective Investors in the United Kingdom

The Notes are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the United Kingdom. For the purposes of this provision:

Consequently no key information document required by Regulation (EU) No 1286/2014 as it forms part of domestic law by virtue of the EUWA (the “UK PRIIPs Regulation”) for offering or selling the Notes or otherwise making them available to retail investors in the UK has been prepared and therefore offering or selling the Notes or otherwise making them available to any retail investor in the UK may be unlawful under the UK PRIIPs Regulation. This prospectus supplement has been prepared on the basis that any offer of Notes in the UK will be made pursuant to an exemption under the UK Prospectus Regulation from the requirement to publish a prospectus for offers of Notes. Neither this prospectus supplement nor the accompanying prospectus is a prospectus for the purposes of the UK Prospectus Regulation.

This prospectus supplement and the accompanying prospectus and any other material in relation to the Notes is only being distributed to, and is directed only at, persons in the United Kingdom who are “qualified investors” (as defined in the UK Prospectus Regulation who are also (i) investment professionals falling within Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005 (as amended, the “Order”), or (ii) high net worth entities or other persons falling within Articles 49(2)(a) to (d) of the Order, or (iii) persons to whom it would otherwise be lawful to distribute it, all such persons together being referred to as “Relevant Persons”. The Notes are only available to, and any invitation, offer or agreement to subscribe, purchase or otherwise acquire such Notes will be engaged in only with, Relevant Persons.

This prospectus supplement and the accompanying prospectus and their contents are confidential and should not be distributed, published or reproduced (in whole or in part) or disclosed by any recipients to any other person in the United Kingdom. Any person in the United Kingdom that is not a Relevant Person should not act or rely on this prospectus supplement and the accompanying prospectus or their contents. The Notes are not being offered to the public in the United Kingdom.

In addition, in the United Kingdom, each underwriter has represented and agreed the Notes may not be offered other than by an underwriter that has only communicated or caused to be communicated and will only communicate or cause to be communicated an invitation or inducement to engage in investment activity (within the meaning of Section 21 of the FSMA) received by it in connection with the issue or sale of the Notes in circumstances in which Section 21(1) of the FSMA does not apply to us; and has complied and will comply with all applicable provisions of the FSMA with respect to anything done by it in relation to the Notes in, from or otherwise involving the United Kingdom.

Notice to Prospective Investors in Japan

The Notes offered by this prospectus supplement have not been and will not be registered under the Financial Instruments and Exchange Act of Japan (Act No. 25 of 1948, as amended, or the FIEA). The Notes offered by this prospectus supplement may not be offered or sold, directly or indirectly, in Japan or to, or for the benefit of any resident of Japan (including any person resident in Japan or any corporation or other entity organized under the laws of Japan) or to others for re-offering or resale, directly or indirectly, in Japan or to, or for the benefit of, a resident of Japan, except pursuant to an exemption from the registration requirements of, and otherwise in compliance with, the FIEA and any other applicable laws, regulations and ministerial guidelines of Japan.

Notice to Prospective Investors in Hong Kong

The Notes may not be offered or sold in Hong Kong by means of any document (except for Notes which are “structured product” as defined in the Securities and Futures Ordinance (Cap. 571 of the Laws of Hong Kong, or the Securities and Futures Ordinance) other than (i) in circumstances which do not constitute an offer to the public within the meaning of the Companies (Winding Up and Miscellaneous Provisions) Ordinance (Cap. 32 of the Laws of Hong Kong, or the Companies (Winding Up and Miscellaneous Provisions) Ordinance), or (ii) to “professional investors” within the meaning of the Securities and Futures Ordinance and any rules made thereunder, or (iii) in other circumstances which do not result in the document being a “prospectus” within the meaning of the Companies (Winding Up and Miscellaneous Provisions) Ordinance, and no advertisement, invitation or document relating to the Notes may be issued or may be in the possession of any person for the purpose of issue (in each case whether in Hong Kong or elsewhere), which is directed at, or the contents of which are likely to be accessed or read by, the public in Hong Kong (except if permitted to do so under the securities laws of Hong Kong) other than with respect to Notes which are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” within the meaning of the Securities and Futures Ordinance and any rules made thereunder.

Notice to Prospective Investors in Singapore

The underwriter has acknowledged that this prospectus supplement has not been and will not be registered as a prospectus with the Monetary Authority of Singapore, or the MAS. Accordingly, the underwriter has represented, warranted and undertaken that it has not offered or sold any Notes or caused the Notes to be made the subject of an invitation for subscription or purchase and will not offer or sell the Notes or cause the Notes to be made the subject of an invitation for subscription or purchase, and has not circulated or distributed, nor will it circulate or distribute, this prospectus supplement or any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the Notes, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act, Chapter 289 of Singapore, or the SFA, (ii) to a relevant person pursuant to Section 275(1), or any person pursuant to Section 275(1A), and in accordance with the conditions specified in Section 275, of the SFA, or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA.

Where the Notes are subscribed or purchased under Section 275 of the SFA by a relevant person which is:

Singapore Securities and Futures Act Product Classification

Solely for the purposes of the underwriter’s obligations pursuant to Sections 309B(1)(a) and 309B(1)(c) of the Securities and Futures Act (Chapter 289 of Singapore), or the SFA, they have determined, and hereby notify all relevant persons (as defined in Section 309A of the SFA) that the Notes are “prescribed capital markets products” (as defined in the Securities and Futures (Capital Markets Products) Regulations 2018) and Excluded Investment Products (as defined in MAS Notice SFA 04-N12: Notice on the Sale of Investment Products) and MAS Notice FAA-N16: Notice on Recommendations on Investment Products.

Notice to Residents of the People’s Republic of China

The underwriter has been advised that the offer of the Notes is not an offer of securities within the meaning of the People’s Republic of China, or PRC, securities laws or other pertinent laws and regulations of the PRC, and the Notes are not being offered or sold and may not be offered or sold, directly or indirectly, in the PRC (for such purposes, not including the Hong Kong and Macau Special Administrative Regions or Taiwan), except as permitted by the securities laws of the PRC.

Notice to Prospective Investors in South Korea

The Notes may not be offered, sold and delivered directly or indirectly, or offered or sold to any person for re-offering or resale, directly or indirectly, in South Korea or to any resident of South Korea except pursuant to the applicable laws and regulations of South Korea, including the Financial Investment Services and Capital Markets Act, or the FSCMA, and the Foreign Exchange Transaction Law and the decrees and regulations thereunder, or the FETL. Furthermore, the purchasers of the Notes comply with all applicable regulatory requirements (including but not limited to requirements under the FETL) in connection with their purchase.

Each underwriter has advised us that it has not offered, sold or delivered the Notes, directly or indirectly, or offered or sold the Notes to any person for re-offering or resale, directly or indirectly, in South Korea or to any resident of South Korea and will not offer, sell or deliver the Notes, directly or indirectly, or offer or sell the Notes to any person for re-offering or resale, directly or indirectly, in South Korea or to any resident of South Korea, except pursuant to an exemption from the registration requirements of, and otherwise in compliance with, the FSCMA, the FETL and other relevant laws and regulations of South Korea.

Notice to Prospective Investors in Taiwan

The Notes have not been and will not be registered with the Financial Supervisory Commission of Taiwan, the Republic of China, or Taiwan, pursuant to relevant securities laws and regulations and may not be offered or sold in Taiwan through a public offering or in any manner which would constitute an offer within the meaning of the Securities and Exchange Act of Taiwan or would otherwise require registration with or the approval of the Financial Supervisory Commission of Taiwan. No person or entity in Taiwan has been authorized to offer or sell the Notes in Taiwan.

Notice to Prospective Investors in Switzerland

Neither this prospectus supplement nor any other offering or marketing material relating to the Notes constitutes a prospectus as such term is understood pursuant to Article 652a or Article 1156 of the Swiss Code of Obligations or a listing prospectus within the meaning of the listing rules of the SIX Swiss Exchange or any other regulated trading facility in Switzerland, and neither this prospectus supplement nor any other offering or marketing material relating to the Notes may be publicly distributed or otherwise made publicly available in Switzerland.

Notice to Prospective Investors in Israel

This prospectus does not constitute a prospectus under the Israeli Securities Law, 5728-1968, and has not been filed with or approved by the Israel Securities Authority. In Israel, this prospectus is being distributed only to, and is directed only at, qualified investors listed in the first addendum, or the Addendum, to the Israeli Securities Law. Qualified investors will be required to submit written confirmation that they fall within the scope of the Addendum.

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LEGAL MATTERS

Certain legal matters regarding the Notes offered by this prospectus supplement will be passed upon for us by Eversheds Sutherland (US) LLP, Washington, D.C. Certain legal matters in connection with the Notes offered hereby will be passed upon for the underwriter by Kirkland & Ellis LLP, Washington, D.C.

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INCORPORATION BY REFERENCE

This prospectus supplement is part of a registration statement that we have filed with the SEC (File No. 333-286240). We are allowed to “incorporate by reference” the information in documents 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):

This prospectus supplement incorporates by reference the documents set forth below that have been previously filed with the SEC:

See “Available Information” for information on how to obtain a copy of these filings.

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AVAILABLE INFORMATION

We have filed with the SEC a registration statement on Form N-2 (File No. 333-286240), together with all amendments and related exhibits, under the Securities Act, with respect to the Notes offered by this prospectus supplement and the accompanying prospectus. The registration statement contains additional information about us and our Notes 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. 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 as soon as practicable after we file such material with the SEC.

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.

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Prospectus

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 (a loan that combines characteristics of traditional first lien senior secured loans and second lien or subordinated loans and that are often referred to by other middle-market lenders as unitranche loans) and other senior secured loans of U.S. middle-market companies. We 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 $75.0 billion of capital under management as of January 1, 2025.

We may offer, from time to time, in one or more offerings or series, together or separately, 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 March 27, 2025 was $15.37 per share. The net asset value of our common stock on December 31, 2024 (the last date prior to the date of this prospectus on which we determined net asset value) was $15.13 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 stockholder inquiries by contacting us at 200 Park Avenue, 25th Floor, New York, NY 10166, 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 March 28, 2025.

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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.

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TABLE OF CONTENTS

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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, 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.”

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PROSPECTUS SUMMARY

This summary highlights information included elsewhere in this prospectus or incorporated by reference. It is not complete and may not contain all of the information that you should consider before making your investment decision. You should carefully read the entire prospectus, the applicable prospectus supplement, and any related free writing prospectus, including the risks of investing in our securities discussed in the section titled “Risk Factors” in this prospectus and the applicable prospectus supplement and any related free writing prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus.

Except as otherwise indicated, the terms:

Golub Capital BDC

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. In addition, for U.S. federal income tax purposes, we have elected to be treated as a regulated investment company, or RIC, under Subchapter M of the Internal Revenue Code of 1986, as amended, or the Code. We were formed in November 2009 to continue and expand the business of our predecessor, Golub Capital Master Funding LLC, which commenced operations in July 2007.

We make investments primarily in one stop (a loan that combines characteristics of traditional first lien senior secured loans and second lien or subordinated loans and that are often referred to by other middle market lenders as unitranche loans) and other senior secured loans of U.S. middle-market companies that are, in most cases, sponsored by private equity firms.

GC Advisors structures these one stop loans as senior secured loans, and we obtain security interests in the assets of the portfolio company that serve as collateral in support of the repayment of these loans. This collateral often takes the form of first-priority liens on the assets of the portfolio company. In many cases, we are the sole lender or we, together with our affiliates, are the sole lenders of one stop loans, which can afford us additional influence over the borrower in terms of monitoring and, if necessary, remediating any underperformance.

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 in industries we believe are resistant to recessions. We also selectively invest in second lien and subordinated (a loan that ranks senior only to a borrower’s equity securities and ranks junior to all of such borrower’s other indebtedness in priority of payment) loans of, and warrants and minority equity securities in, middle-market companies.

We intend to achieve our investment objective by (1) accessing the established loan origination channels developed by Golub Capital, a leading lender to U.S. middle-market companies with over $75.0 billion in capital under management as of January 1, 2025, (2) selecting investments within our core middle-market company focus, (3) partnering with experienced private equity firms, or sponsors, in many cases with whom Golub Capital has invested alongside in the past, (4) implementing the disciplined underwriting standards of Golub Capital and (5) drawing upon the aggregate experience and resources of Golub Capital.

In this prospectus, the term “middle-market” generally refers to companies having earnings before interest, taxes, depreciation and amortization, or EBITDA, of less than $100 million annually.

We seek to create a portfolio that includes primarily one stop and other senior secured loans by primarily investing approximately $10.0 million to $80.0 million of capital, on average, in the securities of U.S. middle-market companies. We expect to selectively invest more than $80.0 million in some of our portfolio companies and generally expect that the size of our individual investments will vary proportionately with the size of our capital base.

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 often 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.

One stop loans include loans to technology companies undergoing strong growth due to new services, increased adoption and/or entry into new markets. We refer to loans to these companies as recurring revenue loans. Other targeted characteristics of recurring revenue businesses include strong customer revenue retention rates, a diversified customer base and backing from growth equity or venture capital firms. In some cases, the borrower’s high revenue growth is supported by a high level of discretionary spending. As part of the underwriting of such loans and consistent with industry practice, we adjust our characterization of the earnings of such borrowers for a reduction or elimination of such discretionary expenses, if appropriate.

Our Adviser

Our investment activities are managed by our investment adviser, GC Advisors. GC Advisors is responsible for sourcing potential investments, conducting research and due diligence on prospective investments and equity sponsors, analyzing investment opportunities, structuring our investments and monitoring our investments and portfolio companies on an ongoing basis. GC Advisors was organized in September 2008 and is a registered investment adviser under the Investment Advisers Act of 1940, as amended, or the Advisers Act. Under the Investment Advisory Agreement, we pay GC Advisors a base management fee and an incentive fee for its services. See “Business — Management Agreements — 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, for a discussion of the base management fee and incentive fee, including the cumulative income incentive fee and the income and capital gains incentive fee, payable by us to GC Advisors and the incentive fee cap. Unlike most closed-end funds whose fees are based on assets net of leverage, our base management fee is based on our average-adjusted gross assets (including leverage but adjusted to exclude cash and cash equivalents so that investors do not pay the base management fee on such assets) and, therefore, GC Advisors benefits when we incur debt or use leverage. For purposes of the Investment Advisory Agreement, cash equivalents means U.S. government securities and commercial paper instruments maturing within 270 days of purchase.

Additionally, under the incentive fee structure, GC Advisors benefits when capital gains are recognized and, because it determines when a holding is sold, GC Advisors controls the timing of the recognition of capital gains. Our board of directors is charged with protecting our interests by monitoring how GC Advisors addresses these, and other conflicts of interest associated with its management services and compensation. While not expected to review or approve each borrowing, our independent directors periodically review GC Advisors’ services and fees as well as its portfolio management decisions and portfolio performance. In connection with these reviews, our independent directors consider whether our fees and expenses (including those related to leverage) remain appropriate. See “Business — Management Agreements” included in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC.

GC Advisors is an affiliate of Golub Capital and pursuant to a staffing agreement, or the Staffing Agreement, Golub Capital LLC makes experienced investment professionals available to GC Advisors and provides access to the senior investment personnel of Golub Capital LLC and its affiliates. The Staffing Agreement provides GC Advisors with access to investment opportunities, which we refer to in the aggregate as deal flow generated by Golub Capital LLC and its affiliates in the ordinary course of their businesses and commits the members of GC Advisors’ investment committee to serve in that capacity. As our investment adviser, GC Advisors is obligated to allocate investment opportunities among us and its other clients fairly and equitably over time in accordance with its allocation policy. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations — Related Party Transactions” 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. However, there can be no assurance that such opportunities will be allocated to us fairly or equitably in the short-term or over time. GC Advisors seeks to capitalize on the significant deal origination, credit underwriting, due diligence, investment structuring, execution, portfolio management and monitoring experience of Golub Capital LLC’s investment professionals.

Our Administrator

An affiliate of GC Advisors, the Administrator, provides the administrative services necessary for us to operate. See “Business — Management Agreements — Administration Agreement” included in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC, for a discussion of the fees and expenses (subject to the review and approval of our independent directors) we are required to reimburse to the Administrator.

About Golub Capital

Golub Capital, founded in 1994, is a leading lender to middle-market companies, with a long track record of investing in senior secured, one stop, second lien and subordinated loans. As of January 1, 2025, Golub Capital had over $75.0 billion of capital under management.

Golub Capital’s middle-market lending group is managed by an eight-member senior management team consisting of Lawrence E. Golub, David B. Golub, Andrew H. Steuerman, Gregory W. Cashman, Spyro G. Alexopoulos, Marc C. Robinson, Robert G. Tuchscherer and Jason J. Van Dussen. As of December 31, 2024, Golub Capital had more than 220 investment professionals supported by more than 775 administrative and back office personnel that focus on operations, finance, legal and compliance, accounting and reporting, marketing, information technology and office management.

Risks Associated with Our Business

Our business is subject to numerous risks, as described in the section titled “Risk Factors” in this prospectus, the applicable prospectus supplement and in the related free writing prospectuses we have authorized for use in connection with a specific offering, and under similar headings in the documents that are incorporated by reference into this prospectus, including the section titled “Risk Factors” included in our most recent Annual Report on Form 10-K, as well as any amendments reflected in subsequent filings with the SEC.

Corporate Information

Our principal executive offices are located at 200 Park Avenue, 25^th Floor, New York, NY 10166, and our telephone number is (212) 750-6060. Our corporate website is located at www.golubcapitalbdc.com. Information on our website is not incorporated into or a part of this prospectus or any related prospectus supplement.

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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 may vary. Actual costs and expenses incurred by investors in shares of our common stock may 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 2024 Debt Securitization (as defined below), the GBDC 3 2022 Debt Securitization (as defined below), prior to their respective terminations on November 18, 2024, the 2018 Debt Securitization (as defined below), the GCIC 2018 Debt Securitization (as defined below), the GBDC 3 2021 Debt Securitization (as defined below), and prior to its termination on December 16, 2024, the GBDC 3 2022-2 Debt Securitization (as defined below), or, collectively, the Debt Securitizations, but includes all of the applicable ongoing fees and expenses of the Debt Securitizations. Whenever this prospectus 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.

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.

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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 could 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 could occur at a price per share that differs from net asset value. See “Dividend Reinvestment Plan” for more information.

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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:

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.

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 have based 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 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 to repay indebtedness (which will be subject to reborrowing).

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 began trading on April 15, 2010 and is currently traded on The Nasdaq Global Select Market under the symbol “GBDC”. The following table lists the high and low closing sale price for our common stock, the closing sale price as a percentage of net asset value, or NAV, and quarterly distributions per share.