We examined the effects of family functioning trajectories on sexual risk behaviors and STI in adolescents. A sample of 850 predominantly (80%) Black adolescents from Michigan, United States, was assessed at baseline, 12, 24, and 36 months postbaseline. Adolescents were from working-class families with a mean age of 14.9 years (SD = 0.64, Range = 13.9 to 16.9) at baseline. Participants completed measures of family functioning at each time point. At 36 months postbaseline, levels of sexual risk behaviors, including sex initiation, unprotected sex, and alcohol or drug use before last sexual intercourse, and STIs were assessed. Latent class growth analysis (LCGA) yielded 4-class solutions for family conflict and parent support. Adolescents with high or increasing family conflict trajectories, and low or decreasing family support trajectories, were at relatively greater risk of sexual risk behaviors and STIs. Yet, the additional trajectories differ across outcomes highlighting the complexities of the role of family functioning on sexual risk behaviors and STIs over time. Multiple Group LCGA indicate some findings vary as a function of gender.