An exigent need exists for HIV prevention intervention research targeting young men who have sex with men (MSM)-a group of young adults that, despite composing the highest and most racially disproportionate rates of HIV incidence, have been least often the focus of behavioral intervention research. This pilot study tested a group-based HIV primary prevention intervention for young MSM to evaluate its initial efficacy, feasibility, and acceptability. Participants were randomized (N = 101; aged 16-20 years) to one of two group-level, HIV and STI education programs: controls participated in a non-interactive, lecture-based program, while intervention participants took part in a highly interactive program tailored to young MSM aged 16-20. Sexual risk and social cognitive outcomes were assessed at baseline, 6-, and 12-weeks post-intervention. Over the entire follow-up period, intervention participants were less likely than controls to engage in any sexual behavior while under the influence of substances (p < .05), and a decreasing trend in unprotected anal sex while under the influence of substances was also observed in this group (p = .08). Follow-up differences between groups on social cognitive outcomes favored the intervention group, though these differences were non-significant. Acceptability ratings were modest. A 6-session behavioral intervention tailored to young MSM, aged 16-20, is feasible, acceptable, and demonstrates evidence of preliminary efficacy in reducing sexual risk, specifically sexual risk while under the influence of substances.