Criminally involved adolescents engage in high levels of risky sexual behavior and alcohol use, and alcohol use may contribute to lack of condom use. Detained adolescents (n = 484) were randomized to (1) a theory-based sexual risk reduction intervention (GPI), (2) the GPI condition with a group-based alcohol risk reduction motivational enhancement therapy component (GPI + GMET), or (3) an information-only control (INFO). All interventions were presented in same-sex groups in single sessions lasting from 2 to 4 hr. Changes to putative theoretical mediators (attitudes, perceived norms, self-efficacy, and intentions) were measured immediately following intervention administration. The primary outcomes were risky sexual behavior and sexual behavior while drinking measured 3 months later (65.1% retention). The GPI + GMET intervention demonstrated superiority over both other conditions in influencing theoretical mediators and over the INFO control in reducing risky sexual behavior. Self-efficacy and intentions were significant mediators between condition and later risky sexual behavior. This study contributes to an understanding of harm reduction among high-risk adolescents and has implications for understanding circumstances in which the inclusion of GMET components may be effective.