Exhibit 4.2

Execution Version

FOURTH SUPPLEMENTAL INDENTURE

between

SIXTH STREET SPECIALTY LENDING, INC.

and

COMPUTERSHARE TRUST COMPANY, N.A.

(AS SUCCESSOR TO WELLS FARGO BANK, NATIONAL ASSOCIATION),

as Trustee

Dated as of August 14, 2023

FOURTH SUPPLEMENTAL INDENTURE

THIS FOURTH SUPPLEMENTAL INDENTURE (this “Fourth Supplemental Indenture”), dated as of August 14, 2023, between Sixth Street Specialty Lending, Inc., a Delaware corporation (the “Company”), and Computershare Trust Company, N.A., as successor to Wells Fargo Bank, National Association, as trustee (the “Trustee”). All capitalized terms used herein shall have the meaning set forth in the Base Indenture (as defined below) unless otherwise defined herein.

RECITALS OF THE COMPANY

The Company and the Trustee executed and delivered an Indenture, dated as of January 22, 2018 (the “Base Indenture” and, as supplemented by this Fourth Supplemental Indenture, collectively, the “Indenture”), to provide for the issuance by the Company from time to time of the Company’s unsecured debentures, notes or other evidences of indebtedness (the “Securities”), to be issued in one or more series as provided in the Indenture.

The Company desires to issue and sell $300,000,000 aggregate principal amount of the Company’s 6.950% Notes due 2028 (the “Notes”).

The Company previously entered into the First Supplemental Indenture, dated as of January 22, 2018 (the “First Supplemental Indenture”), the Second Supplemental Indenture, dated November 1, 2019 (the “Second Supplemental Indenture”) and the Third Supplemental Indenture, dated February 3, 2021 (the “Third Supplemental Indenture”), each of which supplemented the Base Indenture. None of the First Supplemental Indenture, the Second Supplemental Indenture or the Third Supplemental Indenture is applicable to the Notes.

Sections 9.01(v) and 9.01(vii) of the Base Indenture provide that without the consent of Holders of the Securities of any series issued under the Indenture, the Company, when authorized by or pursuant to a Board Resolution, and the Trustee, at any time and from time to

time, may enter into one or more indentures supplemental to the Base Indenture to (i) change or eliminate any of the provisions of the Indenture when there is no Security Outstanding of any series created prior to the execution of a supplemental indenture that is entitled to the benefit of such provision and (ii) establish the form or terms of Securities of any series as permitted by Section 2.01 and Section 3.01 of the Base Indenture.

The Company desires to establish the form and terms of the Notes and to modify, alter, supplement and change certain provisions of the Base Indenture for the benefit of the Holders of the Notes (except as may be provided in a future supplemental indenture to the Indenture (“Future Supplemental Indenture”)).

The Company has duly authorized the execution and delivery of this Fourth Supplemental Indenture to provide for the issuance of the Notes and all acts and things necessary to make this Fourth Supplemental Indenture a valid, binding, and legal obligation of the Company and to constitute a valid agreement of the Company, in accordance with its terms, have been done and performed.

NOW, THEREFORE, THIS INDENTURE WITNESSETH:

For and in consideration of the premises and the purchase of the Notes by the Holders thereof, it is mutually agreed, for the equal and proportionate benefit of all Holders of the Notes, as follows:

ARTICLE I

TERMS OF THE NOTES

Section 1.01. Terms of the Notes. The following terms relating to the Notes are hereby established:

(a) The Notes shall constitute a series of Securities having the title “6.950% Notes due 2028” and shall be designated as Senior Securities under the Indenture. The Notes shall bear a CUSIP number of 83012A AB5 and an ISIN number of US83012AAB52.

(b) The aggregate principal amount of the Notes that may be initially authenticated and delivered under the Indenture (except for Notes authenticated and delivered upon registration of, transfer of, or in exchange for, or in lieu of, other Notes pursuant to Sections 3.04, 3.05, 3.06, 9.06 or 11.07 of the Base Indenture) shall be $300,000,000. Under a Board Resolution, Officers’ Certificate pursuant to Board Resolutions or an indenture supplement, the Company may from time to time, without the consent of the Holders of Notes, issue additional Notes (in any such case “Additional Notes”) having the same ranking and the same interest rate, maturity, CUSIP number and other terms as the Notes; provided that such Additional Notes must either (i) be issued in a “qualified reopening” for U.S. Federal income tax purposes, with no more than a de minimis amount of original issue discount, or otherwise (ii) be part of the same issue as the Notes for U.S. federal income tax purposes. Any Additional Notes and the existing Notes will constitute a single series under the Indenture and all references to the relevant Notes herein shall include the Additional Notes unless the context otherwise requires.

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(c) The entire Outstanding principal amount of the Notes shall be payable on August 14, 2028, unless earlier redeemed or repurchased in accordance with the provisions of this Fourth Supplemental Indenture.

(d) The rate at which the Notes shall bear interest shall be 6.950% per annum (the “Applicable Interest Rate”). The date from which interest shall accrue on the Notes shall be August 14, 2023, or the most recent Interest Payment Date to which interest has been paid or provided for; the Interest Payment Dates for the Notes shall be February 14 and August 14 of each year, commencing February 14, 2024 (if an Interest Payment Date falls on a day that is not a Business Day, then the applicable interest payment will be made on the next succeeding Business Day with the same force and effect as if made on the scheduled Interest Payment Date and no additional interest will accrue as a result of such delayed payment); the initial interest period will be the period from and including August 14, 2023 (or the most recent Interest Payment Date to which interest has been paid or provided for), to, but excluding, the initial Interest Payment Date, and the subsequent interest periods will be the periods from and including an Interest Payment Date to, but excluding, the next Interest Payment Date or the Stated Maturity, as the case may be; the interest so payable, and punctually paid or duly provided for, on any Interest Payment Date, will be paid to the Person in whose name the Note (or one or more predecessor Notes) is registered at 5:00 p.m. New York City time (the “close of business”) on the Regular Record Date for such interest, which shall be January 31 and July 31 (whether or not a Business Day), as the case may be, immediately preceding such Interest Payment Date. Payment of principal of (and premium, if any) and any such interest on the Notes will be made at the Corporate Trust Office of the Paying Agent, which shall initially be the Trustee, in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts; provided, however, that in the case of Notes that are not in global form, at the option of the Company, payment of interest may be made by check mailed to the address of the Person entitled thereto as such address shall appear in the Security Register. Interest on the Notes will be computed on the basis of a 360-day year of twelve 30-day months.

(e) The Notes shall be initially issuable in global form (each such Note, a “Global Note”). The Global Notes and the Trustee’s certificate of authentication thereon shall be substantially in the form of Exhibit A to this Fourth Supplemental Indenture. Each Global Note shall represent the Outstanding Notes as shall be specified therein and each shall provide that it shall represent the aggregate amount of Outstanding Notes from time to time endorsed thereon and that the aggregate amount of Outstanding Notes represented thereby may from time to time be reduced or increased, as appropriate, to reflect exchanges and redemptions. Any endorsement of a Global Note to reflect the amount of any increase or decrease in the amount of Outstanding Notes represented thereby shall be made by the Trustee or the Security Registrar, in accordance with Sections 2.03 and 3.05 of the Base Indenture.

(f) The Depositary Custodian for such Global Notes shall be the Trustee. The Security Registrar with respect to the Global Notes shall be the Trustee.

(g) The Notes shall be defeasible pursuant to Section 14.02 or Section 14.03 of the Base Indenture. Covenant defeasance contained in Section 14.03 of the Base Indenture shall apply to the covenants contained in Sections 10.06, 10.07 and 10.08 of the Indenture.

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(h) The Notes shall be redeemable pursuant to Section 11.01 of the Base Indenture and as follows:

(i) The Notes will be redeemable, in whole or in part, at any time, or from time to time, at the option of the Company, at a Redemption Price (expressed as a percentage of principal amount and rounded to three decimal places), prior to July 14, 2028 (the “Par Call Date”), equal to the greater of the following amounts, plus, in each case, accrued and unpaid interest thereon to the Redemption Date:

provided, however, that if the Company redeems any Notes on or after the Par Call Date, the Redemption Price for the Notes will be equal to 100% of the principal amount of the Notes to be redeemed, plus accrued and unpaid interest thereon to the Redemption Date.

“Treasury Rate” means, with respect to any Redemption Date, the yield determined by the Company in accordance with the following two paragraphs:

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The Company’s actions and determinations in determining the Redemption Price shall be conclusive and binding for all purposes, absent manifest error. The Trustee will not be responsible or liable for determining, confirming, or verifying the Redemption Price.

(ii) Notice of any redemption shall be mailed or electronically delivered (or otherwise transmitted in accordance with the depositary’s procedures) at least 10 days but not more than 60 days before the Redemption Date to each Holder of Notes to be redeemed, at the Holder’s

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address appearing in the Security Register. All notices of redemption shall contain the information set forth in Section 11.04 of the Base Indenture. If the Redemption Price is not known at the time such notice is to be given, the actual Redemption Price, calculated as described in the terms of the Notes, will be set forth in an Officers’ Certificate of the Company delivered to the Trustee no later than two Business Days prior to the Redemption Date.

(iii) Any exercise of the Company’s option to redeem the Notes will be done in compliance with the Investment Company Act, to the extent applicable.

(iv) If the Company elects to redeem only a portion of the Notes, selection of the Notes for redemption shall be made pro rata, by lot or by such other method as the Trustee deems appropriate and fair with the prior written consent of the Company. No Notes of a principal amount of $2,000 or less will be redeemed in part. If any Note is to be redeemed in part only, the notice of redemption that relates to the Note will state the portion of the principal amount of the Note to be redeemed. A new note in a principal amount equal to the unredeemed portion of the Note will be issued in the name of the holder of the Note upon surrender for cancellation of the original Note. For so long as the Notes are held by DTC, Euroclear or Clearstream (or another depositary), the redemption of the Notes shall be done in accordance with the policies and procedures of the depositary.

(v) Unless the Company defaults in payment of the Redemption Price, on and after the Redemption Date, interest will cease to accrue on the Notes called for redemption hereunder.

(i) The Notes shall not be subject to any sinking fund pursuant to Section 12.01 of the Base Indenture.

(j) The Notes shall be issuable in denominations of $2,000 and integral multiples of $1,000 in excess thereof.

(k) Holders of the Notes will not have the option to have the Notes repaid prior to the Stated Maturity other than in accordance with Article Thirteen of the Indenture.

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ARTICLE II

DEFINITIONS AND OTHER PROVISIONS OF GENERAL APPLICATION

Section 2.01. Except as may be provided in a Future Supplemental Indenture, for the benefit of the Holders of the Notes but no other series of Securities under the Indenture, whether now or hereafter issued and Outstanding, Article One of the Base Indenture shall be amended by adding the following defined terms to Section 1.01 of the Base Indenture in appropriate alphabetical sequence, as follows:

“Below Investment Grade Rating Event” means the Notes are downgraded below Investment Grade by all three Rating Agencies on any date from the date of the public notice of an arrangement that results in a Change of Control until the end of the 60-day period following public notice of the occurrence of a Change of Control (which period shall be extended so long as the rating of the Notes is under publicly announced consideration for possible downgrade by any of the Rating Agencies); provided that a Below Investment Grade Rating Event otherwise arising by virtue of a particular reduction in rating shall not be deemed to have occurred in respect of a particular Change of Control (and thus shall not be deemed a Below Investment Grade Rating Event for purposes of the definition of Change of Control Repurchase Event hereunder) if the Rating Agencies making the reduction in rating to which this definition would otherwise apply do not announce or publicly confirm or inform the Company in writing that the reduction was the result, in whole or in part, of any event or circumstance comprised of or arising as a result of, or in respect of, the applicable Change of Control (whether or not the applicable Change of Control shall have occurred at the time of the Below Investment Grade Rating Event).

“Change of Control” means the occurrence of any of the following:

(1) the direct or indirect sale, lease, transfer, conveyance or other disposition (other than by way of merger or consolidation) in one or a series of related transactions, of all or substantially all of the assets of the Company and its Controlled Subsidiaries taken as a whole to any “person” or “group” (as those terms are used in Section 13(d)(3) of the Exchange Act), other than to any Permitted Holders; provided that, for the avoidance of doubt, a pledge of assets pursuant to any secured debt instrument of the Company or its Controlled Subsidiaries shall not be deemed to be any such sale, lease, transfer, conveyance or disposition;

(2) the consummation of any transaction (including, without limitation, any merger or consolidation) the result of which is that any “person” or “group” (as those terms are used in Section 13(d)(3) of the Exchange Act) (other than any Permitted Holders) becomes the “beneficial owner” (as defined in Rules 13d-3 and 13d-5 under the Exchange Act), directly or indirectly, of more than 50% of the outstanding Voting Stock of the Company, measured by voting power rather than number of shares; or

(3) the approval by the Company’s stockholders of any plan or proposal relating to the liquidation or dissolution of the Company.

“Change of Control Repurchase Event” means the occurrence of a Change of Control and a Below Investment Grade Rating Event.

“Corporate Trust Office” means the corporate trust office of the Trustee, at which at any particular time its corporate trust business with respect to this Indenture shall be administered, which office is currently located at Computershare Trust Company, N.A., 1505 Energy Park Drive, St. Paul, MN 55108.

“Depositary” means, with respect to each Note in global form, The Depository Trust Company, until a successor shall have been appointed and becomes such person, and thereafter, Depositary shall mean or include such successor.

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“Fitch” means Fitch Ratings, Inc., also known as Fitch Ratings, or any successor thereto.

“Investment Grade” means a rating of BBB- or better by Fitch (or its equivalent under any successor rating categories of Fitch), BBB- or better by S&P (or its equivalent under any successor rating categories of S&P) and Baa3 or better by Moody’s (or its equivalent under any successor rating categories of Moody’s) (or, in each case, if such Rating Agency ceases to rate the Notes for reasons outside of the Company’s control, the equivalent investment grade credit rating from any Rating Agency selected by the Company as a replacement Rating Agency).

“Moody’s” means Moody’s Investor Service, or any successor thereto.

“Permitted Holders” means (i) the Company, (ii) one or more of the Company’s Controlled Subsidiaries and (iii) Sixth Street Specialty Lending Advisers, LLC or any Affiliate of Sixth Street Specialty Lending Advisers, LLC that is organized under the laws of a jurisdiction located in the United States of America and in the business of managing or advising clients.

“Rating Agency” means (1) each of Fitch, Moody’s and S&P; and (2) if any of Fitch, Moody’s or S&P ceases to rate the Notes or fails to make a rating of the Notes publicly available for reasons outside of the Company’s control, a “nationally recognized statistical rating organization” as defined in Section (3)(a)(62) of the Exchange Act selected by the Company as a replacement agency for Fitch, Moody’s or S&P, or all three, as the case may be.

“S&P” means S&P Global Ratings or any successor thereto.

“Voting Stock” as applied to stock of any Person, means shares, interests, participations or other equivalents in the equity interest (however designated) in such Person having ordinary voting power for the election of a majority of the directors (or the equivalent) of such Person, other than shares, interests, participations or other equivalents having such power only by reason of the occurrence of a contingency.

ARTICLE III

THE TRUSTEE

Section 3.01. Neither the Trustee nor any Paying Agent shall be responsible (or liable) for determining whether any Change of Control or Below Investment Grade Rating Event has occurred and whether any Change of Control offer with respect to the Notes is required. The transferor of any Note shall provide or cause to be provided to the Trustee, upon reasonable request therefore, all information necessary to allow the Trustee to comply with any applicable tax reporting obligations, including without limitation any cost basis reporting obligations under Internal Revenue Code Section 6045. In connection with any proposed exchange of a certificated Note for a Global Note, there shall be provided to the Trustee, upon reasonable request therefore, all information necessary to allow the Trustee to comply with any applicable tax reporting obligations, including without limitation any cost basis reporting obligations under Internal Revenue Code Section 6045. The Trustee may rely on information provided to it and shall have no responsibility (or liability) to verify or ensure the accuracy, correctness, adequacy, or completeness of such information.

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Section 3.02. In no event shall the Trustee be responsible or liable for any failure or delay in the performance of its obligations hereunder arising out of or caused by, directly or indirectly, forces beyond its control, including, without limitation any act or provision of any present or future law or regulation or governmental authority, natural disaster, strikes, work stoppages, accidents, acts of war or terrorism, civil or military disturbances, nuclear or natural catastrophes or acts of God, labor dispute, disease, epidemic or pandemic, quarantine, national emergency and interruptions, loss or malfunctions of utilities, communications or computer (software and hardware) services, communications system failure, malware or ransomware or other unavailability of the Federal Reserve Bank wire or facsimile or telex system or other funds transfer system or other wire or communication facility or unavailability of any securities clearing system; it being understood that the Trustee shall use reasonable efforts that are consistent with accepted practices in the banking industry to resume performance as soon as practicable under the circumstances.

ARTICLE IV

COVENANTS

Section 4.01. Except as may be provided in a Future Supplemental Indenture, for the benefit of the Holders of the Notes but no other series of Securities under the Indenture, whether now or hereafter issued and Outstanding, Article Ten of the Base Indenture shall be amended by replacing Section 10.07 with the following:

“Section 10.07. Section 18(a)(1)(A) of the Investment Company Act.

The Company hereby agrees that for the period of time during which Securities are Outstanding, the Company will not violate, whether or not it is subject thereto, Section 18(a)(1)(A) as modified by Section 61(a) of the Investment Company Act or any successor provisions thereto of the Investment Company Act, but giving effect, in either case, to any exemptive relief granted to the Company by the Commission.”

ARTICLE V

OFFER TO REPURCHASE UPON A CHANGE OF CONTROL REPURCHASE EVENT

Except as may be provided in a Future Supplemental Indenture, for the benefit of the Holders of the Notes but no other series of Securities under the Indenture, whether now or hereafter issued and Outstanding, Article Thirteen of the Base Indenture shall be amended by replacing Sections 13.01 to 13.05 thereto with the following:

“Section 13.01 Change of Control.

If a Change of Control Repurchase Event occurs, unless the Company shall have exercised its right to redeem the Notes in full, the Company shall make an offer to each Holder of the Notes to repurchase all or any part (in minimum denominations of $2,000 and integral multiples of $1,000 principal amount thereabove) of that Holder’s Notes at a repurchase price in cash equal to 100% of the aggregate principal amount of Notes repurchased plus any accrued and unpaid interest on the Notes repurchased to the date of purchase. Within 30 days following any Change of Control Repurchase Event or, at the Company’s option, prior to any Change of Control, but after the public announcement of the Change of Control, the Company will send a

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written notice to each Holder and the Trustee describing the transaction or transactions that constitute or may constitute the Change of Control Repurchase Event and offering to repurchase Notes on the payment date specified in the notice, which date will be no earlier than 30 days and no later than 60 days from the date such notice is sent. The notice shall, if sent prior to the date of consummation of the Change of Control, state that the offer to purchase is conditioned on the Change of Control Repurchase Event occurring on or prior to the payment date specified in the notice. The Company shall comply with the requirements of Rule 14e-1 under the Exchange Act and any other securities laws and regulations thereunder to the extent those laws and regulations are applicable in connection with the repurchase of the Notes as a result of a Change of Control Repurchase Event.

To the extent that the provisions of any securities laws or regulations conflict with this Section 13.01, the Company shall comply with the applicable securities laws and regulations and shall not be deemed to have breached its obligations under this Section 13.01 by virtue of such conflict.

On the Change of Control Repurchase Event payment date, subject to extension if necessary to comply with the provisions of the Investment Company Act, the Company shall, to the extent lawful:

(1) accept for payment all Notes or portions of Notes properly tendered pursuant to its offer;

(2) deposit with the Paying Agent an amount equal to the aggregate purchase price in respect of all Notes or portions of Notes properly tendered; and

(3) deliver or cause to be delivered to the Trustee the Notes properly accepted, together with an Officers’ Certificate stating the aggregate principal amount of Notes being purchased by the Company.

The Paying Agent will promptly remit to each Holder of Notes properly tendered the purchase price for the Notes, and the Trustee will promptly authenticate and mail (or cause to be transferred by book-entry) to each Holder a new Note equal in principal amount to any unpurchased portion of any Notes surrendered; provided that each new Note will be in a minimum principal amount of $2,000 or an integral multiple of $1,000 in excess thereof.

If any Repayment Date upon a Change of Control Repurchase Event falls on a day that is not a Business Day, then the required payment will be made on the next succeeding Business Day and no additional interest will accrue as a result of such delayed payment.

The Company will not be required to make an offer to repurchase the Notes upon a Change of Control Repurchase Event if a third party makes an offer in respect of the Notes in the manner, at the time and otherwise in compliance with the requirements for an offer made by the Company and such third party purchases all Notes properly tendered and not withdrawn under its offer.”

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ARTICLE VI

MISCELLANEOUS

Section 6.01. This Fourth Supplemental Indenture and the Notes shall be governed by and construed in accordance with the laws of the State of New York, without regard to principles of conflicts of laws that would cause the application of laws of another jurisdiction. This Fourth Supplemental Indenture is subject to the provisions of the Trust Indenture Act that are required to be part of the Indenture and shall, to the extent applicable, be governed by such provisions. If any provision of the Indenture limits, qualifies or conflicts with the duties imposed by Section 318(c) of the Trust Indenture Act, the imposed duties will control.

Section 6.02. In case any provision in this Fourth Supplemental Indenture or in the Notes shall be invalid, illegal or unenforceable, the validity, legality and enforceability of the remaining provisions shall not in any way be affected or impaired thereby.

Section 6.03. This Fourth Supplemental Indenture may be executed in any number of counterparts, each of which will be an original, but such counterparts will together constitute but one and the same Fourth Supplemental Indenture. This Fourth Supplemental Indenture shall be valid, binding, and enforceable against a party when executed and delivered by an authorized individual on behalf of the party by means of (i) an original manual signature; (ii) a faxed, scanned, or photocopied manual signature, or (iii) any other electronic signature permitted by the federal Electronic Signatures in Global and National Commerce Act, state enactments of the Uniform Electronic Transactions Act, and/or any other relevant electronic signatures law, including any relevant provisions of the Uniform Commercial Code/UCC (collectively, “Signature Law”), in each case to the extent applicable. Each faxed, scanned, or photocopied manual signature, or other electronic signature, shall for all purposes have the same validity, legal effect, and admissibility in evidence as an original manual signature. Each party hereto shall be entitled to conclusively rely upon, and shall have no liability with respect to, any faxed, scanned, or photocopied manual signature, or other electronic signature, of any other party and shall have no duty to investigate, confirm or otherwise verify the validity or authenticity thereof. For the avoidance of doubt, original manual signatures shall be used for execution or indorsement of writings when required under the UCC or other Signature Law due to the character or intended character of the writings.

Section 6.04. The Base Indenture, as supplemented and amended by this Fourth Supplemental Indenture, is in all respects ratified and confirmed, and the Base Indenture and this Fourth Supplemental Indenture shall be read, taken and construed as one and the same instrument with respect to the Notes. All provisions included in this Fourth Supplemental Indenture supersede any conflicting provisions included in the Base Indenture with respect to the Notes, unless not permitted by law. The Trustee accepts the trusts created by the Indenture, as supplemented by this Fourth Supplemental Indenture, and agrees to perform the same upon the terms and conditions of the Indenture, as supplemented by this Fourth Supplemental Indenture. All of the provisions contained in the Base Indenture in respect of the rights, privileges, protections, limitations of liability, indemnities, immunities, powers, and duties of the Trustee shall be applicable in respect of this Fourth Supplemental Indenture as fully and with like force and effect as though fully set forth in full herein.

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Section 6.05. The provisions of this Fourth Supplemental Indenture shall become effective as of the date hereof.

Section 6.06. Notwithstanding anything else to the contrary herein, the terms and provisions of this Fourth Supplemental Indenture shall apply only to the Notes and shall not apply to any other series of Securities under the Indenture and this Fourth Supplemental Indenture shall not and does not otherwise affect, modify, alter, supplement or change the terms and provisions of any other series of Securities under the Indenture, whether now or hereafter issued and Outstanding.

Section 6.07. The recitals contained herein and in the Notes shall be taken as the statements of the Company, and the Trustee assumes no responsibility for their accuracy, completeness or correctness. The Trustee makes no representations as to and shall not be responsible or liable for the validity or sufficiency of this Fourth Supplemental Indenture, the Notes or any Additional Notes, except that the Trustee represents that it is duly authorized to execute and deliver this Fourth Supplemental Indenture, authenticate the Notes and any Additional Notes and perform its obligations hereunder. The Trustee shall not be responsible or liable for the use or application by the Company of the Notes or any Additional Notes or the proceeds thereof.

Section 6.08. EACH OF THE COMPANY AND THE TRUSTEE HEREBY IRREVOCABLY WAIVES, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE LAW, ANY AND ALL RIGHT TO TRIAL BY JURY IN ANY LEGAL PROCEEDING ARISING OUT OF OR RELATING TO THIS FOURTH SUPPLEMENTAL INDENTURE, THE SECURITIES OR THE TRANSACTION CONTEMPLATED HEREBY.

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IN WITNESS WHEREOF, the parties hereto have caused this Fourth Supplemental Indenture to be duly executed as of the date first above written.

[Signature Page to Fourth Supplemental Indenture]

Execution Version

Exhibit A – Form of Global Note

THIS SECURITY IS A GLOBAL NOTE WITHIN THE MEANING OF THE INDENTURE HEREINAFTER REFERRED TO AND IS REGISTERED IN THE NAME OF THE DEPOSITORY TRUST COMPANY OR A NOMINEE THEREOF. THIS SECURITY MAY NOT BE EXCHANGED IN WHOLE OR IN PART FOR A SECURITY REGISTERED, AND NO TRANSFER OF THIS SECURITY IN WHOLE OR IN PART MAY BE REGISTERED, IN THE NAME OF ANY PERSON OTHER THAN THE DEPOSITORY TRUST COMPANY OR A NOMINEE THEREOF, EXCEPT IN THE LIMITED CIRCUMSTANCES DESCRIBED IN THE INDENTURE.

Unless this certificate is presented by an authorized representative of The Depository Trust Company to the issuer or its agent for registration of transfer, exchange or payment and such certificate issued in exchange for this certificate is registered in the name of Cede & Co., or such other name as requested by an authorized representative of The Depository Trust Company, any transfer, pledge or other use hereof for value or otherwise by or to any person is wrongful, as the registered owner hereof, Cede & Co., has an interest herein.

Sixth Street Specialty Lending, Inc.

6.950% Notes due 2028

Sixth Street Specialty Lending, Inc., a corporation duly organized and existing under the laws of Delaware (herein called the “Company”, which term includes any successor Person under the Indenture hereinafter referred to), for value received, hereby promises to pay to Cede & Co., or registered assigns, the principal sum of THREE HUNDRED MILLION DOLLARS (U.S. $300,000,000), or such other principal sum as shall be set forth in the Schedule of Increases or Decreases attached hereto, on August 14, 2028, and to pay interest thereon from August 14, 2023 or from the most recent Interest Payment Date to which interest has been paid or duly provided for, semi-annually on February 14 and August 14 of each year, commencing February 14, 2024, at the rate of 6.950% per annum, until the principal hereof is paid or made available for payment. The interest so payable, and punctually paid or duly provided for, on any Interest Payment Date will, as provided in such Indenture, be paid to the Person in whose name this Security is registered at 5:00 p.m. New York City time (the “close of business”) on the Regular Record Date for such interest, which shall be January 31 and July 31 (whether or not a Business Day), as the case may be, immediately preceding such Interest Payment Date. Any such interest not so punctually paid or duly provided for will forthwith cease to be payable to the Holder on such Regular Record Date and may either be paid to the Person in whose name this Security is registered at the close of business on a Special Record Date for the payment of such Defaulted Interest to be fixed by the Company, notice whereof shall be given to Holders of Securities of this series not less than 10 days prior to such Special Record Date, or be paid at any

time in any other lawful manner not inconsistent with the requirements of any securities exchange on which the Securities of this series may be listed, and upon such notice as may be required by such exchange, all as more fully provided in said Indenture. This Security may be issued as part of a series.

Payment of the principal of (and premium, if any) and any such interest on this Security will be made at the Corporate Trust Office of the Paying Agent, which shall initially be the Trustee, in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts; provided, however, that at the option of the Company payment of interest may be made by check mailed to the address of the Person entitled thereto as such address shall appear in the Security Register.

Reference is hereby made to the further provisions of this Security set forth on the reverse hereof, which further provisions shall for all purposes have the same effect as if set forth at this place.

Unless the certificate of authentication hereon has been executed by the Trustee referred to on the reverse hereof by manual signature, this Security shall not be entitled to any benefit under the Indenture or be valid or obligatory for any purpose.

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IN WITNESS WHEREOF, the Company has caused this instrument to be duly executed.

Dated:                    

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This is one of the Securities of the series designated therein referred to in the within-mentioned Indenture.

Dated:                    

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[BACK OF NOTE]

Sixth Street Specialty Lending, Inc.

6.950% Notes due 2028

This Security is one of a duly authorized issue of securities of the Company (herein called the “Securities”), issued and to be issued in one or more series under an Indenture, dated as of January 22, 2018 (herein called the “Base Indenture”, which term shall have the meaning assigned to it in such instrument), between the Company and Computershare Trust Company, N.A., as successor to Wells Fargo Bank, National Association, as Trustee (herein called the “Trustee”, which term includes any successor trustee under the Base Indenture), and reference is hereby made to the Base Indenture for a statement of the respective rights, limitations of rights, duties and immunities thereunder of the Company, the Trustee, and the Holders of the Securities and of the terms upon which the Securities are, and are to be, authenticated and delivered, as supplemented by the Fourth Supplemental Indenture, relating to the Securities, dated as of August 14, 2023, by and between the Company and the Trustee (herein called the “Fourth Supplemental Indenture”; and together with the Base Indenture, the “Indenture”). In the event of any conflict between the Base Indenture and the Fourth Supplemental Indenture, the Fourth Supplemental Indenture shall govern and control.

This Security is one of the series designated on the face hereof, initially limited in aggregate principal amount to $300,000,000. Under a Board Resolution, Officers’ Certificate pursuant to Board Resolutions or an indenture supplement, the Company may from time to time, without the consent of the Holders of Securities, issue additional Securities of this series (in any such case “Additional Securities”) having the same ranking and the same interest rate, maturity, CUSIP number and other terms as the Securities, provided that such Additional Securities must either (i) be issued in a “qualified reopening” for U.S. Federal income tax purposes, with no more than a de minimis amount of original issue discount, or otherwise (ii) be part of the same issue as the Securities for U.S. federal income tax purposes. Any Additional Securities and the existing Securities will constitute a single series under the Indenture and all references to the relevant Securities herein shall include the Additional Securities unless the context otherwise requires. The aggregate amount of Outstanding Securities represented hereby may from time to time be reduced or increased, as appropriate, to reflect exchanges and redemptions.

The Securities of this series are subject to redemption in whole or in part, at any time, or from time to time, at the option of the Company, at a Redemption Price (expressed as a percentage of principal amount and rounded to three decimal places), prior to the Par Call Date, equal to the greater of the following amounts, plus, in each case, accrued and unpaid interest thereon to the Redemption Date:

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provided, however, that if the Company redeems any Securities on or after the Par Call Date, the Redemption Price for the Securities will be equal to 100% of the principal amount of the Securities to be redeemed, plus accrued and unpaid interest thereon to the Redemption Date.

“Treasury Rate” means, with respect to any Redemption Date, the yield determined by the Company in accordance with the following two paragraphs:

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The Company’s actions and determinations in determining the Redemption Price shall be conclusive and binding for all purposes, absent manifest error. The Trustee will not be responsible or liable for determining, confirming, or verifying the Redemption Price.

Notice of any redemption shall be mailed or electronically delivered (or otherwise transmitted in accordance with the depositary’s procedures) at least 10 days but not more than 60 days before the Redemption Date to each Holder of Securities to be redeemed, at the Holder’s address appearing in the Security Register. All notices of redemption shall contain the information set forth in Section 11.04 of the Base Indenture.

Any exercise of the Company’s option to redeem the Securities will be done in compliance with the Investment Company Act, to the extent applicable.

If the Company elects to redeem only a portion of the Securities, selection of the Securities for redemption shall be made pro rata, by lot or by such other method as the Trustee deems appropriate and fair with the prior written consent of the Company. No Securities of a principal amount of $2,000 or less will be redeemed in part. If any Security is to be redeemed in part only, the notice of redemption that relates to the Security will state the portion of the principal amount of the Security to be redeemed. A new security in a principal amount equal to the unredeemed portion of the Security will be issued in the name of the holder of the Security upon surrender for cancellation of the original Security. For so long as the Securities are held by DTC, Euroclear or Clearstream (or another depositary), the redemption of the Securities shall be done in accordance with the policies and procedures of the depositary.

Unless the Company defaults in payment of the Redemption Price, on and after the Redemption Date, interest will cease to accrue on the Securities called for redemption.

Holders will have the right to require the Company to repurchase their Securities upon the occurrence of a Change of Control Repurchase Event as set forth in the Indenture.

The Indenture contains provisions for defeasance at any time of the entire indebtedness of this Security or certain restrictive covenants and Events of Default with respect to this Security, in each case upon compliance with certain conditions set forth in the Indenture.

If an Event of Default with respect to Securities of this series shall occur and be continuing (other than Events of Default related to certain events of bankruptcy, insolvency or reorganization as set forth in the Indenture), the principal of the Securities of this series may be

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declared due and payable in the manner and with the effect provided in the Indenture. In the case of certain events of bankruptcy, insolvency or reorganization described in the Indenture, 100% of the principal of and accrued and unpaid interest on the Securities will automatically become due and payable.

The Indenture permits, with certain exceptions as therein provided, the amendment thereof and the modification of the rights and obligations of the Company and the rights of the Holders of the Securities of each series to be affected under the Indenture at any time by the Company and the Trustee with the consent of the Holders of not less than a majority in principal amount of the Securities at the time Outstanding of each series to be affected. The Indenture also contains provisions permitting the Holders of specified percentages in principal amount of the Securities of each series at the time Outstanding, on behalf of the Holders of all Securities of such series, to waive compliance by the Company with certain provisions of the Indenture and certain past defaults under the Indenture and their consequences. Any such consent or waiver by the Holder of this Security shall be conclusive and binding upon such Holder and upon all future Holders of this Security and of any Security issued upon the registration of transfer hereof or in exchange herefor or in lieu hereof, whether or not notation of such consent or waiver is made upon this Security.

As provided in and subject to the provisions of the Indenture, the Holder of this Security shall not have the right to institute any proceeding with respect to the Indenture or for the appointment of a receiver or trustee or for any other remedy thereunder, unless such Holder shall have previously given the Trustee, and a responsible officer of the Trustee has received, written notice of, or a responsible officer of the Trustee has actual knowledge of, a continuing Event of Default with respect to the Securities of this series, the Holders of not less than 25% in principal amount of the Securities of this series at the time Outstanding shall have made written request to the Trustee to institute proceedings in respect of such Event of Default as Trustee and offered the Trustee indemnity against the costs, expenses (including attorneys’ fees and expenses), court costs and liabilities to be incurred in compliance with such request, and the Trustee shall not have received from the Holders of a majority in principal amount of Securities of this series at the time Outstanding a direction inconsistent with such request, and shall have failed to institute any such proceeding, for ninety (90) days after receipt of such notice, request and offer of indemnity. The foregoing shall not apply to any suit instituted by the Holder of this Security for the enforcement of any payment of principal hereof or any premium or interest hereon on or after the respective due dates expressed herein.

No reference herein to the Indenture and no provision of this Security or of the Indenture shall alter or impair the obligation of the Company, which is absolute and unconditional, to pay the principal of and any premium and interest on this Security at the times, place and rate, and in the coin or currency, herein prescribed.

As provided in the Indenture and subject to certain limitations therein set forth, the transfer of this Security is registrable in the Security Register, upon surrender of this Security for registration of transfer at the office or agency of the Company in any place where the principal of and any premium and interest on this Security are payable, duly endorsed by, or accompanied by a written instrument of transfer in form satisfactory to the Company and the Security Registrar duly executed by, the Holder hereof or his attorney duly authorized in writing, and thereupon one or more new Securities of this series and of like tenor, of authorized denominations and for the same aggregate principal amount, will be issued to the designated transferee or transferees.

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The Securities of this series are issuable only in registered form without coupons in denominations of $2,000 and any integral multiples of $1,000 in excess thereof. As provided in the Indenture and subject to certain limitations therein set forth, Securities of this series are exchangeable for a like aggregate principal amount of Securities of this series and of like tenor of a different authorized denomination, as requested by the Holder surrendering the same.

No service charge shall be made for any such registration of transfer or exchange, but the Company or Trustee may require payment of a sum sufficient to cover any tax or other governmental charge payable in connection therewith.

Prior to due presentment of this Security for registration of transfer, the Company, the Trustee and any agent of the Company or the Trustee may treat the Person in whose name this Security is registered as the owner hereof for all purposes, whether or not this Security be overdue, and neither the Company, the Trustee nor any such agent shall be affected by notice to the contrary.

All terms used in this Security which are defined in the Indenture shall have the meanings assigned to them in the Indenture.

To the extent any provision of this Security conflicts with the express provisions of the Indenture, the provisions of the Indenture shall govern and be controlling.

The Indenture and this Security shall be governed by and construed in accordance with the laws of the State of New York, without regard to principles of conflicts of laws.

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Execution Version

ASSIGNMENT FORM

To assign this Note, fill in the form below:

Signature Guarantee*:                                                             

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SCHEDULE OF INCREASES AND DECREASES OF GLOBAL NOTE

The Initial principal amount of this Global Note is $        . The following increases and decreases to this Global Note have been made:

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OPTION OF HOLDER TO ELECT PURCHASE

If you want to elect to have this Note purchased by the Company pursuant to Section 13.01 of the Indenture, check the box below:

☐ Section 13.01

If you want to elect to have only part of the Note purchased by the Company pursuant to Section 13.01 of the Indenture, state the amount you elect to have purchased:

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Exhibit 5.1

August 14, 2023

Sixth Street Specialty Lending, Inc.

2100 McKinney Avenue, Suite 1500

Dallas, TX 75201

Ladies and Gentlemen:

We have acted as counsel to Sixth Street Specialty Lending, Inc., a Delaware corporation (the “Company”), in connection with the Company’s offering pursuant to a registration statement on Form N-2 (333-269222) as amended as of August 7, 2023 for purposes of Rule 430B(f)(2) under the Securities Act of 1933, as amended (the “Securities Act”) (as so amended, including the documents incorporated by reference therein, but excluding Exhibit (d)(3), the “Registration Statement”), and the prospectus, dated August 7, 2023 (including the documents incorporated by reference therein, the “Base Prospectus”), as supplemented by the prospectus supplement, dated August 7, 2023 (including the documents incorporated by reference therein, the “Prospectus Supplement” and, together with the Base Prospectus, the “Prospectus”), of $300,000,000 aggregate principal amount of 6.950% Notes due 2028 (the “Securities”). The Securities were issued pursuant to an indenture dated as of January 22, 2018 (the “Base Indenture”) between the Company and Wells Fargo Bank, National Association, as trustee, as supplemented with respect to the Securities by the Fourth Supplemental Indenture thereto dated as of August 14, 2023 (the “Supplemental Indenture” and, together with the Base Indenture, the “Indenture”) between the Company and Computershare Trust Company, N.A., as successor to Wells Fargo Bank, National Association (the “Trustee”).

In arriving at the opinion expressed below, we have reviewed the following documents:

Cleary Gottlieb Steen & Hamilton LLP or an affiliated entity has an office in each of the locations listed above.

Sixth Street Specialty Lending, Inc., p. 2

In addition, we have reviewed the originals or copies certified or otherwise identified to our satisfaction of all such corporate records of the Company and such other documents, and we have made such investigations of law, as we have deemed appropriate as a basis for the opinion expressed below.

In rendering the opinion expressed below, we have assumed the authenticity of all documents submitted to us as originals and the conformity to the originals of all documents submitted to us as copies. In addition, we have assumed and have not verified the accuracy as to factual matters of each document we have reviewed.

Based on the foregoing, and subject to the further assumptions and qualifications set forth below, it is our opinion that the Securities are the valid, binding and enforceable obligations of the Company, entitled to the benefits of the Indenture.

Insofar as the foregoing opinion relates to the valid existence and good standing of the Company or any subsidiary of the Company, it is based solely on confirmation from public officials. Insofar as the foregoing opinion relates to the validity, binding effect or enforceability of any agreement or obligation of the Company, (x) we have assumed that the Company and each other party to such agreement or obligation has satisfied those legal requirements that are applicable to it to the extent necessary to make such agreement or obligation enforceable against it (except that no such assumption is made as to the Company regarding matters of the law of the State of New York or the General Corporation Law of the State of Delaware that in our experience normally would be applicable to general business entities with respect to such agreement or obligation), (y) we express no opinion with respect to the effect of mandatory choice of law rules and (z) such opinion is subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and to general principles of equity.

The foregoing opinion is limited to the law of the State of New York and the General Corporation Law of the State of Delaware.

Sixth Street Specialty Lending, Inc., p. 3

We hereby consent to the use of our name in the Prospectus Supplement and Final Prospectus under the headings “Legal Matters” as counsel for the Company that has passed on the validity of the Securities, and to the filing of this opinion letter as Exhibit 5.1 to the Company’s Current Report on Form 8-K dated August 14, 2023. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Securities and Exchange Commission thereunder.

The opinion expressed herein is rendered on and as of the date hereof, and we assume no obligation to advise you or any other person, or to make any investigations, as to any legal developments or factual matters arising subsequent to the date hereof that might affect the opinion expressed herein.