A large body of research has identified peer exposure as a key factor driving adolescent substance use. However, findings on the role of sex partners are less robust and mixed. This study aims to fill this gap by examining the independent contribution of close friends' and sex partners' alcohol and marijuana use on adolescents' use of these substances. A secondary data analysis of social network data collected in 2000-2002 from a household sample of African American youth (14-19 years old) in the Bayview and Hunter's Point neighborhoods of San Francisco was conducted. Index participants and their nominated close friends and romantic sex partners (N = 104 triads) self-reported recent alcohol and marijuana use (defined as any use in the past 3 months). Generalized estimated equations were used to estimate the independent association between adolescent's recent substance use and their friend's and sex partner's use. Adolescents with a marijuana-using romantic sex partner had a nearly six-fold higher odds of using marijuana compared to adolescents with a non-using partner, controlling for close friend's marijuana use and other confounders [OR:5.69, 95%CI: 1.94, 16.7]; no association with close friend's marijuana use was found. A similar pattern was observed for alcohol use. Adolescents with an alcohol-using romantic sex partner had increased odds of using alcohol compared to adolescents with a non-using partner, controlling for close friend's alcohol use and other confounders [OR:2.40, 95%CI: 1.02, 5.63]; no association with close friend's alcohol use was found. Romantic sex partners may play a unique and significant role in adolescent substance use. Peer-focused interventions may be more effective if they consider romantic sex partners. Future research should consider the role of romantic sex partners in changing social context related to substance use from adolescence to young adulthood.