Filed Pursuant to Rule 424(b)(2)
Registration No. 333-269222

The information in this prospectus supplement is not complete and may be changed. A registration statement relating to these securities has been filed with and declared effective by the Securities and Exchange Commission. This prospectus supplement and the accompanying prospectus are not an offer to sell these securities and are not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject to Completion

Preliminary Prospectus Supplement dated May 10, 2023

PROSPECTUS SUPPLEMENT

(To Prospectus dated January 13, 2023)

4,500,000 Shares

Common Stock

We are offering for sale 4,500,000 shares of our common stock. We have also granted the underwriters a 30-day option to purchase up to 675,000 additional shares of our common stock.

We are a specialty finance company that has elected to be regulated as a business development company under the Investment Company Act of 1940. We seek to generate current income primarily through direct originations of senior secured loans and, to a lesser extent, originations of mezzanine and unsecured loans and investments in corporate bonds and equity securities.

As of March 31, 2023, our investment portfolio consisted of investments in 127 portfolio companies (including 44 structured credit investments, which include each series of collateralized loan obligation as a separate portfolio company investment) with an aggregate fair value of $2,918.6 million. We intend to continue to pursue an investment strategy focused primarily on direct origination of loans to middle-market companies domiciled in the United States.

We are an externally managed, closed-end, non-diversified management investment company. Sixth Street Specialty Lending Advisers, LLC, or the Adviser, acts as our investment adviser and administrator. We and the Adviser are part of Sixth Street Partners, LLC, a global investment firm with over $65 billion in assets under management as of March 31, 2023.

The companies in our investment portfolio are typically highly leveraged, and, in many cases, our investments in these companies are not rated by any rating agency. If these investments were rated, we believe that most would likely receive a rating of below investment grade (that is, below BBB- or Baa3, which is often referred to as “junk”). Our exposure to below investment grade instruments involves certain risks, including speculation with respect to the borrower’s capacity to pay interest and repay principal. The debt investments in our portfolio generally have a significant portion of principal due at the maturity of the investment, which would result in a substantial loss to us if such borrowers are unable to refinance or repay their debt at maturity.

Substantially all of our debt investments have variable interest rates that reset periodically based on interest rate benchmarks such as the London Interbank Offered Rate, the Euro Interbank Offered Rate, the Canadian Dollar Offered Rate, the Secured Overnight Financing Rate, Sterling Overnight Interbank Average Rate or the Prime Rate. As a result, significant increases in such interest rate benchmarks (or any replacement benchmarks) in the future would make it more difficult for these borrowers to service their obligations under the debt investments that we hold.

Our common stock is traded on the New York Stock Exchange, or NYSE, under the symbol “TSLX.” On May 9, 2023, the last reported sales price of our common stock on the NYSE was $18.19 per share. The net asset value per share of our common stock at March 31, 2023 (the last date prior to the date of this prospectus supplement for which we reported net asset value) was $16.59.

Investing in our common stock involves risks, including the risk of leverage, that are described in the “Risk Factors ” section beginning on page S-6 of this prospectus supplement and page 22 of the accompanying prospectus and the matters discussed in the documents incorporated or deemed to be incorporated by reference in this prospectus supplement and the accompanying prospectus.

Please read this prospectus supplement and the accompanying prospectus, including any information incorporated by reference herein or therein, before investing and keep such documents for future reference. This prospectus supplement and the accompanying prospectus, and the documents incorporated by reference herein or therein, contain important information about us that a prospective investor ought to know before investing in our securities. Information required to be included in a Statement of Additional Information may be found in this prospectus supplement and the accompanying prospectus or incorporated by reference herein or therein. We also file periodic and current reports, proxy statements and other information about us with the Securities and Exchange Commission (the “SEC”). This information is available free of charge by contacting us at 888 7th Avenue, 41st Floor, New York, NY 10106, Attention: TSLX Investor Relations, by emailing us at IRTSLX@sixthstreet.com or visiting our website at http://www.sixthstreetspecialtylending.com. Information on our website is not incorporated into or a part of this prospectus supplement or the accompanying prospectus. The SEC also maintains a website at http://www.sec.gov that contains this information.

The underwriters have agreed to purchase our common stock from us at a price of $             per share, which will result in approximately $            million of net proceeds to us after deducting offering expenses. The underwriters propose to offer the shares of common stock from time to time for sale in negotiated transactions or otherwise, at market prices prevailing at the time of sale, at prices related to such prevailing market prices or at negotiated prices. See “Underwriting.”

We have also granted the underwriters an option to purchase up to an additional            shares from us at the public offering price, less the underwriting discount, within 30 days of the date of this prospectus supplement. If the underwriters exercise this option in full, the total public offering price will be $            , the total underwriting discount (sales load) paid by us will be $            , and total proceeds, before expenses, will be $            .

Neither the SEC nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The shares will be ready for delivery on or about                , 2023.

Joint Book-Running Managers

The date of this prospectus supplement is May     , 2023.

Prospectus Supplement

TABLE OF CONTENTS

Prospectus

TABLE OF CONTENTS

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement and the accompanying prospectus contain 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 identified elsewhere in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein or therein, the following factors, among others, could cause actual results to differ materially from forward-looking statements or historical performance:

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 prospectus supplement or the accompanying prospectus should not be regarded as a representation by us that our plans and objectives will be achieved. These risks and uncertainties include, among other things, those described or identified in “Risk Factors” in this prospectus supplement, the accompanying prospectus and in Part I, Item 1A of our 2022 Annual Report and in Part I, Item 1A of our 1Q 2023 Quarterly Report, any subsequent Quarterly Reports on Form 10-Q or Current Reports on Form 8-K we file after the date of this prospectus supplement, and those discussed in other documents we file with the SEC and elsewhere in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein or therein. You should not place undue reliance on these forward-looking statements, which apply only as of the date of this prospectus supplement. We do not undertake any obligation to update or revise any forward-looking statements or any other information contained herein or therein, except as required by applicable law.

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RISK FACTORS

Investing in our securities involves a number of significant risks. You should carefully consider the risks described in Part I, Item 1A, “Risk Factors,” in our 2022 Annual Report and those set out in the accompanying prospectus before making a decision about investing in our securities. The risks and uncertainties described in Part I, Item 1A, “Risk Factors,” in our 2022 Annual Report and our 1Q 2023 Quarterly Report and discussed in the accompanying prospectus are not the only ones we face. Additional risks and uncertainties not presently known to us or not presently deemed material by us may also materially and adversely affect our business, financial condition and results of operations. In such case, our net asset value and the trading price of our securities could decline, and you may lose all or part of your investment.

Risks Related to Our Common Stock and This Offering

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

We intend to continue paying dividends on a quarterly basis to our stockholders out of assets legally available for distribution. We cannot assure you that we will achieve investment results or maintain a tax status that will allow or require any specified level of cash dividends or year-to-year increases in cash dividends. Our ability to pay dividends might be adversely affected by the impact of one or more of the risk factors described in this prospectus supplement. Due to the asset coverage test applicable to us under the 1940 Act as a BDC or restrictions under our credit facilities, we may be limited in our ability to pay dividends. Although a portion of our expected earnings and dividend distributions will be attributable to net interest income, we do not expect to generate capital gains from the sale of our portfolio investments on a level or uniform basis from quarter to quarter. This may result in substantial fluctuations in our quarterly dividend payments.

In some cases where we receive certain upfront fees in connection with loans we originate, we treat the loan as having OID under applicable accounting and tax regulations, even though we have received the corresponding cash. In other cases, however, we may recognize income before or without receiving the corresponding cash, including in connection with the accretion of OID. For other risks associated with debt obligations treated as having OID, see “Risk Factors—Risks Related to Our Portfolio Company Investments—There are certain risks associated with holding debt obligations that have original issue discount or payment-in-kind interest” in Part I, Item 1A of our 2022 Annual Report.

Therefore, we may be required to make a distribution to our stockholders in order to satisfy the annual distribution requirement necessary to qualify for and maintain RIC tax treatment under Subchapter M of the Code, even though we may not have received the corresponding cash amount. Accordingly, we may have to sell investments at times we would not otherwise consider advantageous, raise additional debt or equity capital or reduce new investment originations to meet these distribution requirements for this purpose. If we are not able to obtain cash from other sources, we may fail to qualify as a RIC and thereby be subject to corporate-level income tax.

To the extent that the amounts distributed by us exceed our current and accumulated earnings and profits, these excess distributions will be treated first as a return of capital to the extent of a stockholder’s tax basis in his or her shares and then as capital gain. Reducing a stockholder’s tax basis will have the effect of increasing his or her gain (or reducing loss) on a subsequent sale of shares.

The part of the Incentive Fee payable by us that relates to our net investment income is computed and paid on income that may include interest that has been accrued but not yet received in cash. If a portfolio company defaults on a loan, it is possible that accrued interest previously used in the calculation of the Incentive Fee will become uncollectible. Consequently, while we may make Incentive Fee payments on income accruals that we may not collect in the future and with respect to which we do not have a clawback right against our Adviser, the amount of accrued income written off in any period will reduce the income in the period in which the write-off is taken and thereby reduce that period’s Incentive Fee payment, if any.

In addition, the middle-market companies in which we intend to invest may be more susceptible to economic downturns than larger operating companies, and therefore may be more likely to default on their payment obligations to us during recessionary periods. Any such defaults could substantially reduce our net investment income available for distribution in the form of dividends to our stockholders.

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Investing in our securities may involve a high degree of risk.

The investments we make in accordance with our investment objective may result in a higher amount of risk than alternative investment options and volatility or loss of principal. Our investments in portfolio companies may be highly speculative and aggressive and, therefore, an investment in our securities may not be suitable for someone with lower risk tolerance.

The market price of our common stock may fluctuate significantly.

The market price and liquidity of the market for shares of our common stock may be significantly affected by numerous factors, some of which are beyond our control and may not be directly related to our operating performance. These factors include:

In the past, following periods of volatility in the market price of a company’s securities, securities class action litigation has often been brought against that company. If our stock price fluctuates significantly, we may be the target of securities litigation in the future. Securities litigation could result in substantial costs and divert management’s attention and resources from our business.

We cannot assure you that the market price of shares of our common stock will not decline.

Shares of closed-end investment companies, including BDCs, frequently trade at a discount from their net asset value and our stock may also be discounted in the market. This characteristic of closed-end investment companies is separate and distinct from the risk that our net asset value per share of common stock may decline. In the past, shares of BDCs, including at times shares of our

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common stock, have traded at prices per share below net asset value per share. We cannot predict whether our common stock will trade at a price per share above, at or below net asset value per share. In addition, if our common stock trades below its net asset value per share, we will generally not be able to sell additional shares of our common stock to the public at its market price without first obtaining the approval of a majority of our stockholders (including a majority of our unaffiliated stockholders) and our Independent Directors for such issuance. See “Risk Factors—Risks Related to Our Business and Structure—Regulations governing our operation as a BDC affect our ability to, and the way in which we, raise additional capital” in Part I, Item 1A of our 2022 Annual Report.

Sales of substantial amounts of our common stock in the public market may have an adverse effect on the market price of our common stock.

Sales of substantial amounts of our common stock, the availability of such common stock for sale or the perception that such sales could occur could adversely affect the prevailing market prices for our common stock. If this occurs, it could impair our ability to raise additional capital through the sale of equity securities should we desire to do so. We cannot predict what effect, if any, future sales of securities or the availability of securities for future sales will have on the market price of our common stock prevailing from time to time.

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

We have adopted a dividend reinvestment plan, pursuant to which we will reinvest all cash dividends and distributions declared by the Board on behalf of investors who do not elect to receive their dividends in cash. As a result, if the Board authorizes, and we declare, a cash dividend or other distribution, then our stockholders who have not opted out of our dividend reinvestment plan will have their cash distributions automatically reinvested in additional common stock, rather than receiving the cash dividend or other distribution. See “ITEM 1. BUSINESS—Dividend Policy” and “ITEM 1. BUSINESS—Dividend Reinvestment Plan” of our 2022 Annual Report for a description of our dividend policy and obligations.

In addition, the number of shares issued pursuant to the dividend reinvestment plan will be determined based on the market price of shares of our common stock, except in circumstances where the market price exceeds our most recently computed net asset value per share, in which case we will issue shares at the greater of (i) the most recently computed net asset value per share and (ii) 95% of the current market price per share or such lesser discount to the current market price per share that still exceeds the most recently computed net asset value per share. Accordingly, participants in the dividend reinvestment plan may receive a greater number of shares of our common stock than the number of shares associated with the market price of our common stock, resulting in dilution for other stockholders. Stockholders that opt out of our dividend reinvestment plan will experience dilution in their ownership percentage of our common stock over time.

Purchases of our common stock by us under the Company 10b5-1 Plan may result in the price of our common stock being higher than the price that otherwise might exist in the open market.

We have entered into an agreement with Goldman Sachs & Co. LLC, which we refer to as the Company 10b5-1 Plan, in accordance with Rules 10b5-1 and 10b-18 under the Exchange Act, under which Goldman Sachs & Co. LLC, as agent for us, may buy up to $50 million of our common stock in the aggregate during the period ending on the earlier of the date on which all the capital committed to the plan has been exhausted or May 31, 2023.

Whether purchases will be made under the Company 10b5-1 Plan and how much will be purchased at any time is uncertain, dependent on prevailing market prices and trading volumes, all of which we cannot predict. These activities may have the effect of maintaining the market price of our common stock or retarding a decline in the market price of the common stock, and, as a result, the price of our common stock may be higher than the price that otherwise might exist in the open market.

Purchases of our common stock by us under the Company 10b5-1 Plan may result in dilution to our net asset value per share.

On August 4, 2015, the Board authorized us to acquire up to $50 million in aggregate of our common stock from time to time over an initial six month period, and has continued to authorize the refreshment of the $50 million amount authorized under and extension of the stock repurchase program prior to its expiration since that time, most recently as of November 1, 2022. Under the Company 10b5-1 Plan, the agent will increase the volume of purchases made as the price of our common stock declines, subject to volume restrictions.

Dilution to our net asset value per share will occur if we purchase shares of our common stock at a price above the net asset value per share, as it would cause a proportionately smaller increase in our stockholders’ interest in our earnings and assets and their voting interest in us than the decrease in our assets resulting from such repurchase.

As a result of any such dilution, our market price per share may decline. The actual dilutive effect will depend on the number of shares of common stock that could be so repurchased, the price and the timing of any repurchases under the Company 10b5-1 Plan.

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FEES AND EXPENSES

The following table is intended to assist you in understanding the costs and expenses that an investor in our common stock will bear directly or indirectly in the twelve months after the date of this prospectus supplement, based on the assumptions set forth below. We caution you that some of the percentages indicated in the table below are estimates and may vary. The following table should not be considered a representation of our future expenses, which may be greater or less than shown. The offering expenses shown assume that we issue 4,500,000 shares of common stock in the offering at the May 9, 2023 closing price of $18.19 and that the underwriters do not exercise their option to purchase any additional shares. Future expenses will depend on many factors, including our use of leverage, which may vary periodically depending on market conditions, our portfolio composition and our Adviser’s assessment of risks and returns. However, our total borrowings are limited under the 1940 Act so that we may not incur any additional leverage if doing so would cause our asset coverage ratio to fall below 150%, as defined in the 1940 Act. Except where the context suggests otherwise, whenever this prospectus supplement or the accompanying prospectus contains a reference to fees or expenses paid by “us,” the “Company” or “Sixth Street Specialty Lending, Inc.” or says that “we” will pay fees or expenses, stockholders will indirectly bear these fees or expenses as investors in Sixth Street Specialty Lending, Inc.

The Management Fee reflected in the table is calculated by determining the ratio that the Management Fee for the three months ended March 31, 2023 bears to our net assets attributable to common stock (rather than our gross assets).

From time to time, our Adviser voluntarily has waived certain Management Fees. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations—Results of Operations—Management Fees” in Part II, Item 2 of our 1Q 2023 Quarterly Report. The above estimates are based on our actual Management Fees for the three months ended March 31, 2023. For the three months ended March 31, 2023, Management Fees of $0.3 million were waived consisting solely of Management Fees pursuant to the Leverage Waiver.

The Adviser intends to waive a portion of the Management Fee payable under the Investment Advisory Agreement by reducing the Management Fee on assets financed using leverage over 200% asset coverage (in other words, over 1.0x debt to equity) (the “Leverage Waiver”). Pursuant to the Leverage Waiver, the Adviser intends to waive the portion of the Management Fee in

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excess of an annual rate of 1.0% (0.250% per quarter) on the average value of the Company’s gross assets as of the end of the two most recently completed calendar quarters that exceeds the product of (i) 200% and (ii) the average value of our net asset value at the end of the two most recently completed calendar quarters. The Adviser waived Management Fees of $0.3 million for the three months ended March 31, 2023 pursuant to the Leverage Waiver. The Adviser did not waive Management Fees for the three months ended March 31, 2022.

The Incentive Fee consists of two parts, as follows:

Pre-Incentive Fee net investment income means dividends, interest and fee income accrued by us during the calendar quarter, minus our operating expenses for the quarter (including the Management Fee, expenses payable under the Administration Agreement to the Administrator, and any interest expense and dividends paid on any issued and outstanding preferred stock, but excluding the Incentive Fee). Pre-Incentive Fee net investment income includes, in the case of investments with a deferred interest feature (such as original issue discount, debt instruments with pay-in-kind interest and zero coupon securities), accrued income that we may not have received in cash. Pre-Incentive Fee net investment income does not include any realized capital gains, realized capital losses or unrealized capital gains or losses.

For purposes of determining whether pre-Incentive Fee net investment income exceeds the hurdle rate, pre-Incentive Fee net investment income is expressed as a rate of return on the value of our net assets at the end of the immediately preceding calendar quarter.

Pre-Incentive Fee net investment income does not include any realized capital gains, realized capital losses or unrealized capital gains or losses. Because of the structure of the Incentive Fee, it is possible that we may pay an Incentive Fee in a quarter in which we incur a loss. For example, if we receive pre-Incentive Fee net investment income in excess of the quarterly minimum hurdle rate, we will pay the applicable Incentive Fee even if we have incurred a loss in that quarter due to realized and unrealized capital losses. In addition, because the quarterly minimum hurdle rate is calculated based on our net assets, decreases in our net assets due to realized or unrealized capital losses in any given quarter may increase the likelihood that the hurdle rate is reached and therefore the likelihood of us paying an Incentive Fee for that quarter. Our net investment income used to calculate this component of the Incentive Fee is also included in the amount of our gross assets used to calculate the Management Fee because gross assets are total assets (including cash received) before deducting liabilities (such as declared dividend payments).

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Section 205(b)(3) of the Advisers Act, as amended, prohibits the Adviser from receiving the payment of fees on unrealized gains until those gains are realized, if ever. There can be no assurance that such unrealized gains will be realized in the future.

See Note 3 to our consolidated financial statements in our 1Q 2023 Quarterly Report and “Agreements and Related Party Transactions— Administration Agreement; Investment Advisory Agreement;” in Part I, Item 1 of our 1Q 2023 Quarterly Report.

From time to time, our Adviser has voluntarily waived certain Incentive Fees. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations—Results of Operations—Incentive Fees” in Part II, Item 2 of our 1Q 2023 Quarterly Report. The above estimates do not reflect or assume any such waivers.

Example

The following example demonstrates the projected dollar amount of total cumulative expenses over various periods with respect to a hypothetical investment in our common stock. In calculating the following expense amounts, we have assumed we would have no additional leverage and that our annual operating expenses would remain at the levels set forth in the table above. The Incentive Fee payable in the example below assumes that the Incentive Fee is 17.5% for all relevant periods. Transaction expenses are not included in the following example.

The foregoing table is to assist you in understanding the various costs and expenses that an investor in our common stock will bear directly or indirectly. While the example assumes, as required by the SEC, a 5% annual return, our performance will vary and may result in a return greater or less than 5%. Because the income portion of the Incentive Fee under the Investment Advisory Agreement is unlikely to be significant assuming a 5% annual return, the example assumes that the 5% annual return will be generated entirely through the realization of capital gains on our assets and, as a result, will trigger the payment of the capital gains portion of the Incentive Fee under the Investment Advisory Agreement. The income portion of the Incentive Fee under the Investment Advisory Agreement, which, assuming a 5% annual return, would either not be payable or have an immaterial impact on the expense amounts shown above, is not included in the example. If we achieve sufficient returns on our investments, including through the realization of capital gains, to trigger an Incentive Fee of a material amount, our expenses, and returns to our investors, would be higher. In addition, while the example assumes reinvestment of all dividends and distributions at net asset value, under certain circumstances, reinvestment of dividends and other distributions under our dividend reinvestment plan may occur at a price per share that differs from net asset value. See “Dividend Reinvestment Plan” in the accompanying prospectus for additional information regarding our dividend reinvestment plan.

This example and the expenses in the table above should not be considered a representation of our future expenses, and actual expenses (including the cost of debt, if any, and other expenses) may be greater or less than those shown.

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USE OF PROCEEDS

We estimate that the net proceeds we will receive from the sale of 4,500,000 shares of our common stock in this offering will be approximately $                million (or approximately $                million if the underwriters fully exercise their option to purchase additional shares), in each case after deducting the underwriting discounts and commissions and estimated offering expenses payable by us.

We intend to use 100% of the net proceeds of this offering to pay down a portion of the outstanding indebtedness on the Revolving Credit Facility on or about the date of the closing of this offering. As of March 31, 2023, aggregate commitments under the facility were $1.585 billion. However, through reborrowing under the Revolving Credit Facility, we intend to make new investments in accordance with our investment objective and strategies described in this prospectus supplement and the accompanying prospectus. The stated maturity date of the Revolving Credit Facility is April 23, 2027 with respect to $1.510 billion of commitments, January 31, 2025 for $25.0 million of commitments, and February 4, 2026 for $50.0 million of commitments. As of March 31, 2023, amounts drawn under the facility bear interest at either the applicable reference rate plus an applicable credit spread adjustment, plus a margin of either 1.75% or 1.875%, or the base rate plus a margin of either 0.75% or 0.875%. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations—Financial Condition, Liquidity and Capital Resources—Revolving Credit Facility” in Part II, Item 2 of our 1Q 2023 Quarterly Report.

Affiliates of certain underwriters are lenders under the Revolving Credit Facility. Accordingly, affiliates of certain of the underwriters may receive more than 5% of the proceeds of this offering to the extent the proceeds are used to pay down a portion of the outstanding indebtedness under the Revolving Credit Facility.

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CAPITALIZATION

The following table sets forth our consolidated capitalization at March 31, 2023. You should read this table together with “Use of Proceeds” described in this prospectus supplement and our most recent balance sheet included in our 1Q 2023 Quarterly Report incorporated by reference in this prospectus supplement.

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UNDERWRITING

Wells Fargo Securities, LLC is acting as representative of each of the underwriters named below. Subject to the terms and conditions set forth in an underwriting agreement among us and the underwriters, we have agreed to sell to the underwriters, and each of the underwriters has agreed, severally and not jointly, to purchase from us, the number of shares of common stock set forth opposite its name below.

Subject to the terms and conditions set forth in the underwriting agreement, the underwriters have agreed, severally and not jointly, to purchase all of the shares sold under the underwriting agreement if any of these shares are purchased. If an underwriter defaults, the underwriting agreement provides that the purchase commitments of the nondefaulting underwriters may be increased or the underwriting agreement may be terminated.

We have agreed to indemnify the underwriters against certain liabilities, including liabilities under the Securities Act of 1933, as amended (the “Securities Act”), or to contribute to payments the underwriters may be required to make in respect of those liabilities.

The underwriters are offering the shares, subject to prior sale, when, as and if issued to and accepted by them, subject to approval of legal matters by their counsel, including the validity of the shares, and other conditions contained in the underwriting agreement, such as the receipt by the underwriters of officers’ certificates and legal opinions. The underwriters reserve the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

The underwriters may offer the shares of common stock from time to time for sale in one or more transactions on the NYSE, in the over-the-counter market, through negotiated transactions or otherwise at market prices prevailing at the time of sale, at prices related to prevailing market prices or at negotiated prices. The underwriters may effect such transactions by selling shares of common stock to or through dealers, and such dealers may receive compensation in the form of discounts, concessions or commissions from the underwriters and/or purchasers of shares of common stock for whom they may act as agents or to whom they may sell as principal.

The expenses of the offering, not including the underwriting discount, are estimated at $     and are payable by us.

Underwriters’ Option

We have granted the underwriters an option to buy, within 30 days from the date of this prospectus supplement, up to 675,000 additional shares of common stock. If any shares are purchased pursuant to this option, the underwriters will purchase shares in approximately the same proportion as shown in the table above. If any additional shares of common stock are purchased, the underwriters will offer the additional shares on the same terms as those on which the shares are being offered.

No Sales of Similar Securities

Subject to certain exceptions, we have agreed not to directly or indirectly, (i) offer, pledge, sell, contract to sell, grant any option for the sale of or otherwise transfer or dispose of, directly or indirectly, or file with the SEC a registration statement under the Securities Act relating to, any shares of our common stock or securities convertible into or exchangeable or exercisable for any shares of our common stock (except for any “shelf” registration statements filed by us), or publicly disclose the intention to make any offer, sale, pledge, disposition or filing, or (ii) enter into any swap or other arrangement that transfers all or a portion of the economic consequences associated with the ownership of any shares of common stock or any such other securities (regardless of whether any of these transactions are to be settled by the delivery of shares of common stock or such other securities, in cash or otherwise), in each case without the prior written consent of Wells Fargo Securities, LLC for a period of 30 days after the date of this prospectus supplement.

Our Adviser, our directors and certain of our executive officers have entered into lock-up agreements with the underwriters prior to the commencement of this offering pursuant to which each of these persons or entities, with limited exceptions, for a period of 30 days after the date of this prospectus supplement, may not, subject to certain exceptions, without the prior written consent of Wells Fargo Securities, LLC, (1) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, or otherwise transfer or dispose of, directly or indirectly, any shares of our common stock or any securities convertible into or exercisable or exchangeable for our common stock (including, without limitation, common stock or such other securities which may be deemed to be beneficially owned by such directors, executive officers, managers and members in

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accordance with the rules and regulations of the SEC and securities which may be issued upon exercise of a stock option or warrant), or publicly disclose the intention to make any offer, sale, pledge or disposition, or (2) enter into any swap or other agreement that transfers, in whole or in part, any of the economic consequences of ownership of the common stock or such other securities, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of common stock or such other securities, in cash or otherwise, or (3) make any demand for or exercise any right with respect to the registration of any shares of our common stock or any security convertible into or exercisable or exchangeable for our common stock.

NYSE Listing

The shares are listed on the NYSE under the symbol “TSLX.”

Price Stabilization, Short Positions

In connection with this offering, the underwriters may engage in stabilizing transactions, which may involve making bids for, purchasing and selling shares of common stock in the open market for the purpose of preventing or retarding a decline in the market price of the common stock while this offering is in progress. These stabilizing transactions may include making short sales of the common stock, which involves the sale by the underwriters of greater number of shares of common stock than they are required to purchase in this offering, and purchasing shares of common stock on the open market to cover positions created by short sales. Short sales may be “covered” shorts, which are short positions in an amount not greater than the underwriters’ option to purchase additional shares from us, or may be “naked” shorts, which are short positions in excess of that amount. The underwriters may close out any covered short position either by exercising their option to purchase additional shares, in whole or in part, or by purchasing shares in the open market. In making this determination, the underwriters will consider, among other things, the price of shares available for purchase in the open market compared to the price at which the underwriters may purchase shares through their option. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the common stock in the open market that could adversely affect investors who purchase in this offering. To the extent that the underwriters create a naked short position, they will purchase shares in the open market to cover the position.

The underwriters have advised us that, pursuant to Regulation M of the Securities Act, they may also engage in other activities that stabilize, maintain or otherwise affect the price of the common stock, including the imposition of penalty bids. This means that if the representatives of the underwriters purchase common stock in the open market in stabilizing transactions or to cover short sales, the representatives can require the underwriters that sold those shares as part of this offering to repay the sales load received by them.

These activities may have the effect of raising or maintaining the market price of our common stock or preventing or retarding a decline in the market price of our common stock, and, as a result, the price of our common stock may be higher than the price that otherwise might exist in the open market. If the underwriters commence these activities, they may discontinue them at any time. The underwriters may carry out these transactions on the NYSE, in the over-the-counter market or otherwise.

Neither we nor any of the underwriters make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of our common stock. In addition, neither we nor any of the underwriters make any representation that the representatives will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

Electronic Distribution

The underwriters may make prospectuses available in electronic (PDF) format. A prospectus in electronic (PDF) format may be made available on a web site maintained by the underwriters, and the underwriters may distribute such prospectuses electronically. The underwriters may allocate a limited number of shares for sale to their online brokerage customers.

Other Relationships

The underwriters and their respective affiliates are full service financial institutions engaged in various activities, which may include sales and trading, commercial and investment banking, advisory, investment management, investment research, principal investment, hedging, market making, brokerage, valuation services and other financial and non-financial activities and services. Certain of the underwriters and their respective affiliates have provided, currently provide, and may in the future provide, a variety of these services to us and to persons and entities with relationships with us, for which they received or will receive customary fees and expenses. Certain of the underwriters and their affiliates were underwriters in connection with our IPO and our subsequent debt and equity offerings, for which they received customary fees. Additionally, affiliates of certain underwriters are lenders under the Revolving Credit Facility.

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In the ordinary course of their various business activities, the underwriters and their respective affiliates, officers, directors and employees may purchase, sell or hold a broad array of investments and actively trade securities, derivatives, loans, commodities, currencies, credit default swaps and other financial instruments for their own account and for the accounts of their customers, and such investment and trading activities may involve or relate to our assets, securities or instruments (directly, as collateral securing other obligations or otherwise) or persons and entities with relationships with us. Certain of the underwriters and their affiliates that have a lending relationship with us routinely hedge their credit exposure to us consistent with their customary risk management policies. Typically, such underwriters and their affiliates would hedge such exposure by entering into transactions which consist of either the purchase of credit default swaps or the creation of short positions in our securities. Any such credit default swaps or short positions could adversely affect future trading prices of our securities. The underwriters and their respective affiliates may also communicate independent investment recommendations, market color or trading ideas or publish or express independent research views in respect of such assets, securities or instruments and may at any time hold, or recommend to clients that they should acquire, long or short positions in such assets, securities and instruments.

We intend to use the net proceeds of this offering to pay down a portion of the outstanding indebtedness under the Revolving Credit Facility and for general corporate purposes. Affiliates of certain underwriters are lenders under the Revolving Credit Facility. Accordingly, affiliates of certain of the underwriters may receive more than 5% of the proceeds of this offering to the extent the proceeds are used to pay down a portion of the outstanding indebtedness under the Revolving Credit Facility.

Goldman Sachs & Co. LLC is also acting as our agent for the Company stock repurchase 10b5-1 Plan.

Principal Business Addresses

The principal business address of Wells Fargo Securities, LLC is 550 South Tryon Street, Charlotte, North Carolina 28202.

Other Jurisdictions

Other than in the United States, no action has been taken by us or the underwriters that would permit a public offering of the securities offered by this prospectus supplement in any jurisdiction where action for that purpose is required. The securities offered by this prospectus supplement may not be offered or sold, directly or indirectly, nor may this prospectus supplement or any other offering material or advertisements in connection with the offer and sale of any such securities be distributed or published in any jurisdiction, except under circumstances that will result in compliance with the applicable rules and regulations of that jurisdiction. Persons into whose possession this prospectus supplement comes are advised to inform themselves about and to observe any restrictions relating to the offering and the distribution of this prospectus supplement. This prospectus supplement does not constitute an offer to sell or a solicitation of an offer to buy any securities offered by this prospectus supplement in any jurisdiction in which such an offer or a solicitation is unlawful.

Notice to Prospective Investors in the Dubai International Financial Centre

This prospectus supplement relates to an Exempt Offer in accordance with the Offered Securities Rules of the Dubai Financial Services Authority, or DFSA. This prospectus supplement is intended for distribution only to persons in the in the Dubai International Financial Centre, or DIFC of a type specified in the Offered Securities Rules of the DFSA. It must not be delivered to, or relied on by, any other person in the DIFC. The DFSA has no responsibility for reviewing or verifying any documents in connection with Exempt Offers. The DFSA has not approved this prospectus supplement nor taken steps to verify the information set forth herein and has no responsibility for the prospectus supplement. The common stock to which this prospectus supplement relates may be illiquid or subject to restrictions on their resale. Prospective purchasers of the common stock offered should conduct their own due diligence on the common stock. If you do not understand the contents of this prospectus supplement you should consult an authorized financial advisor.

Notice to Prospective Investors in Hong Kong

The common stock has not been offered or sold and will not be offered or sold in Hong Kong, by means of any document, other than (a) to “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571 of the Laws of Hong Kong) (“Securities and Futures Ordinance”) and any rules made thereunder; or (b) in other circumstances which do not result in the document being a “prospectus” as defined in the Companies (Winding Up and Miscellaneous Provisions) Ordinance (Cap. 32 of the Laws of Hong Kong) (“Companies (Winding Up and Miscellaneous Provisions) Ordinance”) or which do not constitute an offer to the public within the meaning of the Companies (Winding Up and Miscellaneous Provisions) Ordinance. No advertisement, invitation or document relating to the common stock has been or may be issued or has been or may be in the possession of any person for the purposes of issue, whether in Hong Kong or elsewhere, which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do so under the securities laws of Hong Kong) other than with respect to common stock which are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” as defined in the Securities and Futures Ordinance and any rules made thereunder.

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Notice to Prospective Investors in Canada

This prospectus constitutes an “exempt offering document” as defined in and for the purposes of applicable Canadian securities laws. No prospectus has been filed with any securities commission or similar regulatory authority in Canada in connection with the offer and sale of the shares. No securities commission or similar regulatory authority in Canada has reviewed or in any way passed upon this prospectus or on the merits of the shares and any representation to the contrary is an offence.

Canadian investors are advised that this prospectus has been prepared in reliance on section 3A.3 of National Instrument 33-105 Underwriting Conflicts (“NI 33-105”). Pursuant to section 3A.3 of NI 33-105, the company and the underwriters in the offering are exempt from the requirement to provide Canadian investors with certain conflicts of interest disclosure pertaining to “connected issuer” and/or “related issuer” relationships as would otherwise be required pursuant to subsection 2.1(1) of NI 33-105.

Resale Restrictions

The offer and sale of the shares in Canada is being made on a private placement basis only and is exempt from the requirement that the company prepares and files a prospectus under applicable Canadian securities laws. Any resale of shares by a Canadian investor in this offering must be made in accordance with applicable Canadian securities laws, which may vary depending on the relevant jurisdiction, and which may require resales to be made in accordance with Canadian prospectus requirements, a statutory exemption from the prospectus requirements, in a transaction exempt from the prospectus requirements or otherwise under a discretionary exemption from the prospectus requirements granted by the applicable local Canadian securities regulatory authority. These resale restrictions may under certain circumstances apply to resales of the shares outside of Canada.

Representations of Purchasers

Each Canadian investor who purchases the shares will be deemed to have represented to the company, the underwriters and to each dealer from whom a purchase confirmation is received, as applicable, that the investor is (i) purchasing as principal, or is deemed to be purchasing as principal in accordance with applicable Canadian securities laws; (ii) an “accredited investor” as such term is defined in section 1.1 of National Instrument 45-106 Prospectus Exemptions or, in Ontario, as such term is defined in section 73.3(1) of the Securities Act (Ontario); and (iii) a “permitted client” as such term is defined in section 1.1 of National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations.

Taxation and Eligibility for Investment

Any discussion of taxation and related matters contained in this prospectus does not purport to be a comprehensive description of all of the tax considerations that may be relevant to a Canadian investor when deciding to purchase the shares and, in particular, does not address any Canadian tax considerations. No representation or warranty is hereby made as to the tax consequences to a resident, or deemed resident, of Canada of an investment in the shares or with respect to the eligibility of the Notes for investment by such investor under relevant Canadian federal and provincial legislation and regulations.

Rights of Action for Damages or Rescission

Securities legislation in certain of the Canadian jurisdictions provides certain purchasers of securities pursuant to an offering memorandum (such as this prospectus), including where the distribution involves an “eligible foreign security” as such term is defined in Ontario Securities Commission Rule 45-501 Ontario Prospectus and Registration Exemptions and in Multilateral Instrument 45-107 Listing Representation and Statutory Rights of Action Disclosure Exemptions, as applicable, with a remedy for damages or rescission, or both, in addition to any other rights they may have at law, where the offering memorandum, or other offering document that constitutes an offering memorandum, and any amendment thereto, contains a “misrepresentation” as defined under applicable Canadian securities laws. These remedies, or notice with respect to these remedies, must be exercised or delivered, as the case may be, by the purchaser within the time limits prescribed under, and are subject to limitations and defences under, applicable Canadian securities legislation. In addition, these remedies are in addition to and without derogation from any other right or remedy available at law to the investor.

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Language of Documents

Upon receipt of this document, each Canadian investor hereby confirms that it has expressly requested that all documents evidencing or relating in any way to the sale of the securities described herein (including for greater certainty any purchase confirmation or any notice) be drawn up in the English language only. Par la réception de ce document, chaque investisseur canadien confirme par les présentes qu’il a expressément exigé que tous les documents faisant foi ou se rapportant de quelque manière que ce soit à la vente des valeurs mobilières décrites aux présentes (incluant, pour plus de certitude, toute confirmation d’achat ou tout avis) soient rédigés en anglais seulement.

LEGAL MATTERS

Certain legal matters in connection with the offering will be passed upon for us by Cleary Gottlieb Steen & Hamilton LLP, New York, New York. In addition, Proskauer Rose LLP and Morris, Nichols, Arsht & Tunnell LLP will pass on certain legal matters for us. Cleary Gottlieb Steen & Hamilton LLP and Proskauer Rose LLP also represent the Adviser. Certain legal matters in connection with the offering will be passed upon for the underwriters by Ropes & Gray LLP.

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INFORMATION INCORPORATED BY REFERENCE

This prospectus supplement is part of a registration statement that we have filed with the SEC. Pursuant to the Small Business Credit Availability Act, we are allowed to “incorporate by reference” the information that we file with the SEC, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is considered to comprise a part of this prospectus supplement from the date we file that document. Any reports filed by us with the SEC before the date that any offering of any securities by means of this prospectus supplement and the accompanying prospectus is terminated will automatically update and, where applicable, supersede any information contained in this prospectus supplement or incorporated by reference in this prospectus supplement. This prospectus supplement incorporates by reference the documents listed below:

We incorporate by reference into this prospectus supplement additional documents that we may file with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, as amended, or the Exchange Act, until all of the securities offered by this prospectus supplement and the accompanying prospectus have been sold or we otherwise terminate the offering of these securities; provided, however, the information “furnished” under Item 2.02 or Item 7.01 of Form 8-K or other information “furnished” to the SEC which is not deemed filed is not incorporated by reference in this prospectus supplement and any accompanying prospectus. Information that we file with the SEC will automatically update and may supersede information in this prospectus supplement, the accompanying prospectus and information previously filed with the SEC.

We will provide without charge to each person, including any beneficial owner, to whom a copy of this prospectus supplement is delivered, upon written request of any such person, a copy of any or all of the documents that has been or may be incorporated by reference into this prospectus supplement (excluding certain exhibits to the documents) at no cost. Any such request may be made by contacting us in writing at the following address:

Sixth Street Specialty Lending, Inc.

888 7th Avenue, 41st Floor

New York, NY 10106

Attention: TSLXInvestor Relations

Email: IRTSLX@sixthstreet.com.

You should rely only on the information incorporated by reference or provided in this prospectus supplement or accompanying prospectus. We have not authorized anyone to provide you with different or additional information, and you should not rely on such information if you receive it. We are not making an offer of or soliciting an offer to buy, any securities in any state or other jurisdiction where such offer or sale is not permitted. You should not assume that the information in this prospectus supplement or in the documents incorporated by reference is accurate as of any date other than the date on the front of this prospectus supplement or those documents.

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