Black men who have sex with men (MSM) in the United States experience disproportionately high rates of HIV and other sexually transmitted infections (STIs); however, the number of evidence-based interventions for Black MSM is limited. This study evaluated the efficacy of Many Men, Many Voices (3MV), a small-group HIV/STI prevention intervention developed by Black MSM-serving community-based organizations and a university-based HIV/STI prevention and training program. The study sample included 338 Black MSM of HIV-negative or unknown HIV serostatus residing in New York city. Participants were randomly assigned to the 3MV intervention condition (n = 164) or wait-list comparison condition (n = 174). Relative to comparison participants, 3MV participants reported significantly greater reductions in any unprotected anal intercourse with casual male partners; a trend for consistent condom use during receptive anal intercourse with casual male partners; and significantly greater reductions in the number of male sex partners and greater increases in HIV testing. This study is the first randomized trial to demonstrate the efficacy of an HIV/STI prevention intervention for Black MSM.