Page 1

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31 , 2020 or 0 TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission file number 001-36364 Sixth Street Specialty Lending , Inc. ( Exact Name of Registrant as Specified in Its Charter ) Delaware ( State or other jurisdiction of incorporation or organization ) 2100 McKinney Avenue , Suite 1500 , Dallas , TX ( Address of principal executive offices ) Title of each class Common Stock , par value $ 0.01 per share 75201 ( Zip Code ) Registrant's Telephone Number , Including Area Code : ( 469 ) 621-3001 Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) TSLX Securities registered pursuant to Section 12 ( g ) of the Act : None ( Title of class ) 27-3380000 ( I.R.S. Employer Identification No. ) YES > NO ☐ NO > Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . YES □ Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . YES > NO ☐ Large accelerated filer Non - Accelerated filer Name of each exchange on which registered The New York Stock Exchange Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . YES □ NO É Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See the definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Accelerated filer Smaller reporting company Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . 000 Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . < Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Act ) . YES □ NO The aggregate market value of the voting stock held by non - affiliates of the registrant on June 30 , 2020 , based on the closing price on that date of $ 16.49 on The New York Stock Exchange , was approximately $ 1,061,937,084 . The number of shares of the registrant's common stock , $ .01 par value per share , outstanding at February 17 , 2021 was 67,896,113 . DOCUMENTS INCORPORATED BY REFERENCE Portions of the Registrant's proxy statement for the 2020 annual meeting of stockholders are incorporated by reference in Part III .