Exhibit 99.1 K FIRST QUARTER 2024 EARNINGS RESULTS Sixth Street Specialty Lending, Inc. Reports First Quarter Results; Declares a Second Quarter Base Dividend Per Share of $0.46, and a First Quarter Supplemental Dividend Per Share of $0.06. NEW YORK — May 1, 2024 — Sixth Street Specialty Lending, Inc. (NYSE: TSLX, or Net Investment Income Per Share the “Company”) today reported net investment income of $0.59 per share and net Q1 2024: $0.59 income of $0.53 per share for the first quarter ended March 31, 202 ...