UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of report (Date of earliest event reported): February 9, 2022

BAIN CAPITAL SPECIALTY FINANCE, INC.

(Exact name of Registrant as Specified in Its Charter)

Registrant’s telephone number, including area code: (617) 516-2000

N/A

(Former Name or Former Address, if Changed Since Last Report)

Securities registered pursuant to Section 12(b) of the Act:

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ¨

On February 9, 2022, Bain Capital Specialty Finance, Inc. (the “Company”), and an entity advised by Amberstone Co., Ltd. (“Amberstone”), a credit focused investment manager that advises institutional investors, committed capital to a newly formed joint venture, Bain Capital Senior Loan Program, LLC (“SLP”). Pursuant to an amended and restated limited liability company agreement (the “LLC Agreement”) between the Company and Amberstone, each such party has a 50% economic ownership interest in SLP. Total initial capital commitments to SLP are $358 million, with each party expected to maintain their pro rata proportionate share for each capital contribution. SLP will seek to invest primarily in senior secured first lien loans of U.S. borrowers. Investment decisions and all other material decisions in respect of SLP must be approved by representatives of the Company and Amberstone.

The description above includes only a summary of the material terms included in the LLC Agreement and is qualified in its entirety by reference to a copy of the LLC Agreement, a copy of which will be filed as an exhibit to the Company’s Annual Report on Form 10-K for the year ended December 31, 2021.

(d) Exhibits.

99.1 Bain Capital Specialty Finance, Inc. Press Release, dated February 14, 2022

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, Bain Capital Specialty Finance, Inc. has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.