This randomized controlled superiority study will be conducted during 2023-2025. The eligible participants are adult people living with HIV (PLWH) who are newly diagnosed with early syphilis. Participants will be randomized in a 1:1 ratio to receive single-dose benzathine penicillin G (BPG) (2.4 MU intramuscularly once) plus doxycycline (100 mg orally twice daily for 7 days) or single-dose BPG. The primary outcome is serologic response, defined as a decline of rapid plasma reagin (RPR) titer by 4-fold or greater, at week 24 and week 48; and the secondary outcomes include microbiologic response of syphilis and bacterial sexually transmitted infections (STIs) assessed by nucleic-acid amplification test (NAAT) at week 4.