Sixth Street Specialty Lending, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 10-Q

(Mark One)

For the quarterly period ended September 30,2026

OR

For the transition period from to

Commission File Number 001-36364

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (469)621-3001

Not applicable

(Former name, former address and former fiscal year, if changed since last report)

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

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes ☒ No☐

Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit such files). Yes ☒ No☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

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.☐

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yes☐ No☒

The number of shares of the registrant’s common stock, $.01 par value per share, outstanding at November 3, 2026 was 95,713,159.

SIXTH STREET SPECIALTY LENDING, INC.

CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This report contains forward-looking statements that involve substantial risks and uncertainties. These forward-looking statements are not historical facts, but rather are based on current expectations, estimates and projections about us, our current and prospective portfolio investments, our industry, our beliefs, and our assumptions. Words such as “anticipates,” “expects,” “intends,” “plans,” “believes,” “seeks,” “estimates,” “would,” “should,” “targets,” “projects,” and variations of these words and similar expressions are intended to identify forward-looking statements. These statements are not guarantees of future performance and are subject to risks, uncertainties, and other factors, some of which are beyond our control and difficult to predict, that could cause actual results to differ materially from those expressed or forecasted in the forward-looking statements.

In addition to factors previously identified elsewhere in the reports and other documents Sixth Street Specialty Lending, Inc. (the "Company", "we", "us", or "our") has filed with the Securities and Exchange Commission (the “SEC”), the following factors, among others, could cause actual results to differ materially from forward-looking statements or historical performance:

•

an economic downturn could impair our portfolio companies’ abilities to continue to operate, which could lead to the loss of some or all of our investments in those portfolio companies;

•

such an economic downturn could disproportionately impact the companies in which we have invested and others that we intend to target for investment, potentially causing us to experience a decrease in investment opportunities and diminished demand for capital from these companies;

•

such an economic downturn could also impact availability and pricing of our financing;

•

an inability to access the capital markets could impair our ability to raise capital and our investment activities;

•

inflation could negatively impact our business, including our ability to access the debt markets on favorable terms, or could negatively impact our portfolio companies;

•

changes in political, economic or industry conditions, the interest rate environment or conditions affecting the financial and capital markets, including the effects of trade policies, government regulation and geopolitical conflicts may adversely affect our operations and the operations of our portfolio companies; and

•

the risks, uncertainties and other factors we identify in the section entitled “Risk Factors” in this report, in our Annual Report on Form 10-K for the year ended December 31, 2025, filed with the SEC on February 12, 2026, and elsewhere in our filings with the SEC.

Although we believe that the assumptions on which these forward-looking statements are based are reasonable, some of those assumptions are based on the work of third parties and any of those assumptions could prove to be inaccurate; as a result, forward-looking statements based on those assumptions also could prove to be inaccurate. In light of these and other uncertainties, the inclusion of a projection or forward-looking statement in this report should not be regarded as a representation by us that our plans and objectives will be achieved. You should not place undue reliance on these forward-looking statements, which apply only as of the date of this report. We do not undertake any obligation to update or revise any forward-looking statements or any other information contained herein, except as required by applicable law.

The “TSLX” and “TAO” marks are marks of Sixth Street.