UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, DC 20549 FORM 10-Q (Mark One) ☒ QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934. For the quarterly period ended September 30, 2023 OR ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934. For the transition period from to Commission File Number 001-36364 Sixth Street Specialty Lending, Inc. (Exact name of registrant as specified in its charter) Delaware 27-3380000 (State ...