BACKGROUND: Consistent evidence shows stigma impedes healthcare access in people living with HIV (PLWH) and men who have sex with men (MSM). We evaluated the impact of a stigma reduction training for providers whose design was informed by direct observation of their clinical behaviors obtained through visits by incognito standardized patient (SP). SETTING: We conducted this study in in sexually transmitted infection clinics in Guangzhou, China. METHODS: This pilot cluster randomized control trial assessed the feasibility, acceptability, and preliminary efficacy of an intervention whose design was informed by a baseline round of incognito visits in which SPs presented standardized cases to consenting doctors. By randomly varying the HIV status and sexual orientation of each case, we could quantify stigma as differences in care quality across scenarios. We then conducted a follow-up round of SP visits and assessed impact using linear fixed effects regression. RESULTS: Feasibility and acceptability among the 55 provider participants was high, with no adverse visit events. The training improved testing for HIV negative MSM (0.05 percentage points [PP], 95% CI,-0.24, 0.33) and diagnostic effort in HIV positive MSM (0.23 standard deviation [SD] improvement, 95% CI, -0.92, 1.37). Patient-centered care only improved for HIV positive straight cases (SD, 0.57; 95% CI, -0.39, 1.53). All estimates lacked statistical precision, an expected outcome of a pilot RCT. CONCLUSIONS: Our training reduced stigma in in several domains of care, but least of all for PLWH, suggesting that future trainings should include more clinical content to strengthen clinical skills in PLWH management.