Sixth Street Specialty Lending, Inc.

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

Filed by the Registrant ☒ Filed by a Party other than the Registrant ☐

Check the appropriate box:

(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):

To Our Stockholders:

We are pleased to invite you to attend a Special Meeting of Stockholders of Sixth Street Specialty Lending, Inc. to be held on May 21, 2026, at 10:00 a.m., Eastern Time, at the offices of Simpson Thacher & Bartlett LLP, 425 Lexington Avenue, 30th Floor, New York, NY 10017.

The following pages include a formal notice of the meeting and our proxy statement. The Notice of Internet Availability of Proxy Materials that you received by mail and this proxy statement describe the matter on the agenda for the meeting. At the meeting, you will be asked to consider and vote upon a proposal to authorize the Company, with approval of its Board of Directors, to sell or otherwise issue shares of its common stock at a price below its then-current net asset value per share in one or more offerings, subject to certain conditions as set forth herein, including that the number of shares issued does not exceed 25% of its then-outstanding common stock immediately prior to each such offering.

Please read these materials so that you will know what we plan to do at the meeting. It is important that your shares be represented at the special meeting, regardless of whether you plan to attend the meeting in person. Please vote your shares as soon as possible through any of the voting options available to you as described in the Notice of Internet Availability of Proxy Materials and this proxy statement.

You will also receive separate proxy materials for the Annual Meeting of Stockholders, which will be held at the same place and on the same date stated above, but not at the same time. Please be certain to use each proxy card that you receive from us to vote separately for the matters to be discussed at each meeting.

On behalf of management and our Board of Directors, we thank you for your continued support of Sixth Street Specialty Lending, Inc.

Sincerely,

/s/ Robert (“Bo”) Stanley

Robert (“Bo”) Stanley

Chief Executive Officer

Important Notice Regarding the Availability of Proxy Material for the Special Meeting of Stockholders to Be Held on May 21, 2026.

Our proxy statement is available online at www.proxyvote.com.

The following information applicable to the Special Meeting of Stockholders may be found in the proxy statement and accompanying proxy card:

Notice of a Special Meeting of Stockholders

TO OUR STOCKHOLDERS:

NOTICE IS HEREBY GIVEN that a Special Meeting of Stockholders of Sixth Street Specialty Lending, Inc. will be held at the offices of Simpson Thacher & Bartlett LLP, 425 Lexington Avenue, 30th Floor, New York, NY 10017 on May 21, 2026, at 10:00 a.m., Eastern Time. The Special Meeting of Stockholders is being held for the following purpose:

1. To consider and vote upon a proposal to authorize the Company to sell or otherwise issue shares of its common stock at a price below its then-current net asset value per share in one or more offerings, in each case subject to the approval of its Board of Directors and subject to certain conditions as set forth herein, including that the number of shares issued does not exceed 25% of its then-outstanding common stock immediately prior to each such offering.

Only stockholders of record at the close of business on March 31, 2026, are entitled to notice of and to vote at the Special Meeting of Stockholders or any postponement or adjournment thereof.

We are furnishing a proxy statement and proxy card to our stockholders on the internet, rather than mailing printed copies of those materials to each stockholder. If you received a Notice of Internet Availability of Proxy Materials by mail, you will not receive a printed copy of the proxy statement and proxy card unless you request them. Instead, the Notice of Internet Availability of Proxy Materials will instruct you as to how you may access and review the proxy statement, and vote your proxy, on the internet.

Your vote is extremely important to us. Whether or not you plan to attend the Special Meeting of Stockholders, we encourage you to vote your proxy on the internet by following the instructions provided on the Notice of Internet Availability of Proxy Materials or proxy card. Stockholders may also request from us free of charge hard copies of the proxy statement and proxy card by following the instructions on the Notice of Internet Availability of Proxy Materials. In the event there are not sufficient votes for a quorum at the time of the Special Meeting, the Special Meeting may be adjourned in order to permit further solicitation of proxies by the Company.

THE BOARD OF DIRECTORS, INCLUDING THE INDEPENDENT DIRECTORS, UNANIMOUSLY RECOMMENDS THAT YOU VOTE “FOR” THE PROPOSAL.

By Order of the Board of Directors,

/s/ Anton Brett

Anton Brett

Chief Compliance Officer and Secretary

YOUR VOTE IS IMPORTANT! WE HOPE YOU WILL VOTE OVER THE INTERNET, BY PHONE OR BY MARKING, SIGNING AND RETURNING YOUR PROXY CARD AS SOON AS POSSIBLE, WHETHER OR NOT YOU PLAN TO ATTEND THE MEETING. EVEN IF YOU VOTE YOUR SHARES PRIOR TO THE MEETING, YOU STILL MAY ATTEND THE MEETING AND VOTE YOUR SHARES IN PERSON.