Black women are at disproportionately greater risk for HIV and sexually transmitted infections than women of other ethnic/racial backgrounds. Alcohol use may further elevate the risk of HIV/sexually transmitted infection acquisition and transmission. A random-assignment parallel-group comparative treatment efficacy trial was conducted with random assignment to 1 of 3 conditions. The sample comprised 560 Black or African American women aged 18–24 years who reported recent unprotected vaginal or anal sex and recent alcohol use. Participants were recruited from community settings in Atlanta, Georgia, from January 2012 to February 2014. A Group Motivational Enhancement Therapy module was designed to complement a Centers for Disease Control and Prevention–designated evidence-based intervention (Horizons) to reduce sexual risk behaviors, alcohol use, and sexually transmitted infections, with 3 comparison groups: (1) Horizons + Group Motivational Enhancement Therapy intervention, (2) Horizons + General Health Promotion intervention, and (3) enhanced standard of care. Outcome measures included safe sex (abstinence or 100% condom use); condom nonuse; proportion of condom use during sexual episodes; incident chlamydia, gonorrhea, and trichomonas infections; and problematic alcohol use measured by Alcohol Use Disorders Identification Test score. Treatment effects were estimated using an intention-to-treat protocol‒generalized estimating equations with logistic regression for binomial outcomes and Poisson regression for count outcomes. Analyses were conducted between October 2018 and October 2019. Participants assigned to Horizons + Group Motivational Enhancement Therapy had greater odds of safe sex (AOR=1.45, 95% CI=1.04, 2.02, p=0.03), greater proportion of condom use (AOR=1.68, 95% CI=1.18, 2.41, p=0.004), and lower odds of condom nonuse (AOR=0.57, 95% CI=0.38, 0.83, p=0.004). Both interventions had lower odds of problematic alcohol use (Horizons: AOR=0.57, 95% CI=0.39, 0.85, p=0.006; Horizons + Group Motivational Enhancement Therapy: AOR=0.61, 95% CI=0.41, 0.90, p=0.01). Complementing an evidence-based HIV prevention intervention with Group Motivational Enhancement Therapy may increase safer sexual behaviors and concomitantly reduce alcohol use among young Black women who consume alcohol. This study is registered at www.clinicaltrials.gov NCT01553682.