Sixth Street Specialty Lending, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K

For the fiscal year ended December 31,2025

OR

Commission file number 001-36364

(Exact Name of Registrant as Specified in Its Charter)

Registrant’s Telephone Number, Including Area Code: (469)621-3001

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

Securities registered pursuant to Section 12(g) of the Act: None

(Title of class)

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes ☒ No ☐

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act. Yes ☐No ☒

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 has filed a report on and attestation to its management’s assessment of the effectiveness of its internal control over financial reporting under Section 404(b) of the Sarbanes-Oxley Act (15 U.S.C. 7262(b)) by the registered public accounting firm that prepared or issued its audit report.☒

If securities are registered pursuant to Section 12(b) of the Act, indicate by check mark whether the financial statements of the registrant included in the filing reflect the correction of an error to previously issued financial statements.☐

Indicate by check mark whether any of those error corrections are restatements that required a recovery analysis of incentive-based compensation received by any of the registrant’s executive officers during the relevant recovery period pursuant to §240.10D-1(b).☐

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

The aggregate market value of the voting stock held by non-affiliates of the registrant on June 30, 2025, based on the closing price on that date of $23.81 on The New York Stock Exchange, was approximately $2,158,484,033. The number of shares of the registrant’s common stock, $.01 par value per share, outstanding at February 12, 2026 was 94,705,150.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the Registrant’s proxy statement for the 2026 annual meeting of stockholders are incorporated by reference in Part III.

Auditor Firm Id:185 Auditor Name: KPMG LLPAuditor Location: New York, New York

SIXTH STREET SPECIALTY LENDING, INC.

Index to Annual Report on Form 10-K for

Year Ended December 31, 2025

CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This Annual 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. has filed with the Securities and Exchange Commission, or SEC, the following factors, among others, could cause actual results to differ materially from forward-looking statements or historical performance:

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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;

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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;

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

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

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

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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 and government regulation; and

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the risks, uncertainties and other factors we identify in the section entitled “Risk Factors” in this Annual Report 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 Annual 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 Annual 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 safe harbor provisions of Section 21E of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), which preclude civil liability for certain forward-looking statements, do not apply to the forward-looking statements in this Annual Report because we are an investment company.

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

Summary of Risk Factors

The following is a summary of the principal risks that could adversely affect our business, operations and financial results, which should be read in conjunction with the complete discussion of risk factors we face, which are set forth in “Item 1A. Risk Factors.”

Risks Related to Our Business and Structure

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We are dependent upon management personnel of the Adviser, Sixth Street and their affiliates for our future success.

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We are subject to significant regulations governing our operation as a BDC, which affect our ability to, and the way in which we, raise additional capital. Changes in regulation could adversely affect our business.

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We borrow money, which magnifies the potential for gain or loss and increases the risk of investing in us.

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We may invest through joint ventures, partnerships or other special purpose vehicles and our investments through these vehicles may entail greater risks, or risks that we otherwise would not incur, if we otherwise made such investments directly.

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We operate in a highly competitive market for investment opportunities.

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If we are unable to source investments, access financing or manage future growth effectively, we may be unable to achieve our investment objective.

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Even in the event the value of your investment declines, the Management Fee and, in certain circumstances, the Incentive Fee will still be payable to the Adviser.

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To the extent that we do not realize income or choose not to retain after-tax realized net capital gains, we will have a greater need for additional capital to fund our investments and operating expenses.

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We will be subject to corporate-level U.S. federal income tax if we are unable to maintain our qualification as a RIC under Subchapter M of the Code, including as a result of our failure to satisfy the RIC distribution requirements.

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We can be expected to retain some income and capital gains in excess of what is permissible for excise tax purposes and such amounts will be subject to 4% U.S. federal excise tax.

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Our Adviser and its affiliates, officers and employees may face certain conflicts of interest.

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Our ability to enter into transactions with our affiliates is restricted.

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Our Adviser can resign on 60 days’ notice. We may not be able to find a suitable replacement within that time, resulting in a disruption in our operations and a loss of the benefits from our relationship with Sixth Street.

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The Adviser’s liability is limited under the Investment Advisory Agreement, and we are required to indemnify the Adviser against certain liabilities, which may lead the Adviser to act in a riskier manner on our behalf than it would when acting for its own account.

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Any failure to maintain our status as a BDC would reduce our operating flexibility.

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We incur significant costs as a result of being a publicly traded company.

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Provisions of the General Corporation Law of the State of Delaware and our certificate of incorporation and bylaws could deter takeover attempts and have an adverse effect on the price of our common stock.

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Certain investors are limited in their ability to make significant investments in us.

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Cybersecurity risks and cyber incidents may adversely affect our business or those of our portfolio companies.

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Our Board may change our investment objective, operating policies and strategies without prior notice or stockholder approval.

Risks Related to Economic Conditions

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The current state of the economy and financial markets increases the likelihood of adverse effects on our financial position and results of operations.

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Economic recessions or downturns could impair our portfolio companies and harm our operating results.

Risks Related to Our Portfolio Company Investments

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Our investments are very risky and highly speculative.

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The value of most of our portfolio securities will not have a readily available market price and we value these securities at fair value as determined in good faith by our Board, which valuation is inherently subjective, may not reflect what we may actually realize for the sale of the investment and could result in a conflict of interest with the Adviser.

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The lack of liquidity in our investments may adversely affect our business.

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Our portfolio may be focused on a limited number of portfolio companies or industries, which will subject us to a risk of significant loss if any of these companies defaults on its obligations under any of its debt instruments or if there is a downturn in a particular industry.

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We may securitize certain of our investments, which may subject us to certain structured financing risks.

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Because we generally do not hold controlling interests in our portfolio companies, we may not be in a position to exercise control over those portfolio companies or prevent decisions by management of those portfolio companies that could decrease the value of our investments.

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We may not be able to realize expected returns on our invested capital.

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By originating loans to companies that are experiencing significant financial or business difficulties, we may be exposed to distressed lending risks.

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Our portfolio companies may incur debt or issue equity securities that rank equally with, or senior to, our investments in those companies and we may be exposed to special risks associated with bankruptcy cases.

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Our failure to make follow-on investments in our portfolio companies could impair the value of our investments.

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Any acquisitions or strategic investments that we pursue are subject to risks and uncertainties.

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We cannot guarantee that we will be able to obtain various required licenses in U.S. states or in any other jurisdiction where they may be required in the future.

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Our investments in foreign companies may involve significant risks in addition to the risks inherent in U.S. investments.

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We expose ourselves to risks when we engage in hedging transactions.

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The market structure applicable to derivatives imposed by the Dodd-Frank Act may affect our ability to use over-the-counter (“OTC”) derivatives for hedging purposes.

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Our portfolio investments may present special tax issues, and there are certain risks associated with holding debt obligations that have original issue discount or payment-in-kind interest.

Risks Related to Our Securities

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There is a risk that investors in our common stock may not receive dividends or that our dividends may not grow over time.

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Investing in our securities may involve a high degree of risk and the market price of our common stock may fluctuate significantly and could decline.

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Our stockholders will experience dilution in their ownership percentage if they opt out of our dividend reinvestment plan.

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Purchases of our common stock by us under the Company 10b5-1 Plan may result in dilution to our net asset value per share and the price of our common stock being higher than the price that otherwise might exist in the open market.

General Risk Factors

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We are highly dependent on information systems and systems failures could significantly disrupt our business, which may, in turn, negatively affect the market price of our common stock and our ability to pay dividend.

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Changes in laws or regulations governing our operations may adversely affect our business.

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We are subject to risks related to sustainability matters.