Research examining associations between unhealthy weight and sexual risk behaviors derives primarily from cross-sectional studies conducted at varying ages during adolescence. The present study examined longitudinal associations between adolescent sexual risk behaviors and Body Mass Index (BMI), the latter measured prospectively from childhood through adolescence. Data were drawn from an 8095 individuals between the ages of 14 and 19 at last assessment, including 1768 Hispanic (52% female), 2659 non-Hispanic Black (Black; 50% female), and 3668 non-Hispanic, non-Black (predominantly White; 49% female). Childhood BMI predicted increased sexual risk-taking during adolescence for Black and White youth, and increases in BMI predicted increased sexual risk-taking for White females and Black males. The findings highlight differential patterns of association between BMI and sexual risk behaviors for Hispanic, Black, and White females and males with implications for prevention discussed.