Despite the centrality of dating relationships for teens, it is unclear whether the influence of romantic partners' alcohol use on adolescents' under-age drinking is distinct from the influence of peers and parents. To address this gap, this study used longitudinal data from a population-based sample of 825 adolescents (49% male, 51% female), ages 12 to 19. Adolescents completed a survey using laptops for privacy, and a parent completed a survey separately. Ordinary least squares and logistic regression models assessed alcohol use frequency and alcohol problems and included dating partners' drinking, adolescents' prior drinking, peers' drinking, parents' substance use, parental monitoring, and sociodemographic background characteristics. Alcohol use frequency and alcohol problems were influenced by dating partners' alcohol use and dating partners' influence was stronger on older adolescents and male adolescents. The study results are useful for public health messaging and prevention efforts by demonstrating the influence of parents, peers, and dating partners on teens' alcohol use.