ObjectiveLGBTQ-affirmative cognitive-behavioral therapy (CBT) addresses minority stress to improve sexual minority individuals’ mental and behavioral health. This treatment has never been tested in high-stigma contexts like China using online delivery. MethodChinese young sexual minority men (n = 120; ages 16–30; HIV-negative; reporting depression and/or anxiety symptoms and past-90-day HIV-transmission-risk behavior), were randomized to receive 10 sessions of culturally adapted asynchronous LGBTQ-affirmative internet-based CBT (ICBT) or weekly assessments only. The primary outcome included HIV-transmission-risk behavior (i.e., past-30-day condomless anal sex). Secondary outcomes included HIV social-cognitive mechanisms (e.g., condom use self-efficacy), mental health (e.g., depression), and behavioral health (e.g., alcohol use), as well as minority stress (e.g., acceptance concerns), and universal (e.g., emotion regulation) mechanisms at baseline and 4- and 8-month follow-up. Moderation analyses examined treatment efficacy as a function of baseline stigma experiences and session completion. ResultsCompared to assessment only, LGBTQ-affirmative ICBT did not yield greater reductions in HIV-transmission-risk behavior or social-cognitive mechanisms. However, LGBTQ-affirmative ICBT yielded greater improvements in depression (d = −0.50, d = −0.63) and anxiety (d = −0.51, d = −0.49) at 4- and 8-month follow-up, respectively; alcohol use (d = −0.40) at 8-month follow-up; and certain minority stress (e.g., internalized stigma) and universal (i.e., emotion dysregulation) mechanisms compared to assessment only. LGBTQ-affirmative ICBT was more efficacious for reducing HIV-transmission-risk behavior for participants with lower internalized stigma (d = 0.42). Greater session completion predicted greater reductions in suicidality and rumination. ConclusionsLGBTQ-affirmative ICBT demonstrates preliminary efficacy for Chinese young sexual minority men. Findings can inform future interventions for young sexual minority men in contexts with limited affirmative supports.