UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

(RULE 14a-101)

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No. 1)

Filed by the Registrant  ☒

Filed by a Party other than the Registrant  ☐

Check the appropriate box:

Sixth Street Specialty Lending, Inc.

(Name of Registrant as Specified in Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check all boxes that apply):

AMENDMENT NO. 1 TO THE PROXY STATEMENT

DATED APRIL 9, 2026 FOR THE

SPECIAL MEETING OF STOCKHOLDERS TO BE HELD ON MAY 21, 2026

EXPLANATORY NOTE

This Amendment No. 1 to Schedule 14A (“Amendment No. 1”) is being filed to amend Sixth Street Specialty Lending, Inc.’s definitive proxy statement for its 2026 Special Meeting of Shareholders (the “Meeting”), which was filed with the Securities and Exchange Commission on April 9, 2026 (the “Proxy Statement”). This Amendment No. 1 amends the disclosures provided in the section “Security Ownership of Certain Beneficial Owners and Management” as set forth on page 5 of the Proxy Statement in order to correct the amount of shares held by certain directors as of March 31, 2026. There is no change to the total number of shares held by all directors and officers as a group. All other items of the Proxy Statement are incorporated herein by reference without changes.

Except as specifically discussed in this Explanatory Note, this Amendment No. 1 does not otherwise modify or update any other disclosures presented in the Proxy Statement. In addition, this Amendment No. 1 does not reflect events occurring after the date of the Proxy Statement or modify or update disclosures that may have been affected by subsequent events.

CHANGE TO THE PROXY STATEMENT

The information under the heading “Security Ownership of Certain Beneficial Owners and Management” on page 5 of the Proxy Statement is amended and restated to read as follows:

SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

Beneficial ownership is determined in accordance with the rules and regulations of the U.S. Securities and Exchange Commission (the “SEC”). These rules generally provide that a person is the beneficial owner of securities if the person has or shares the power to vote or direct the voting thereof, or to dispose or direct the disposition thereof or has the right to acquire these powers within 60 days. The following table sets forth, as of March 31, 2026, the beneficial ownership as indicated in the Company’s books and records of each current director, the nominees for director, the Company’s Named Executive Officers and the Company’s executive officers and directors as a group. Based upon Schedule 13D or Schedule 13G filings with the SEC, the Company is not aware of any person that beneficially owns 5% or more of the outstanding shares of the Company’s common stock.

The percentage ownership is based on 95,019,600 shares of common stock outstanding as of March 31, 2026. To the Company’s knowledge, except as indicated in the footnotes to the table, each of the stockholders listed below has sole voting and/or investment power with respect to shares beneficially owned by such stockholder. Unless otherwise indicated by footnote, the address for each listed individual is 2100 McKinney Avenue, Suite 1500, Dallas, TX 75201.