The quality of romantic relationships formed during early adulthood has critical implications for physical and psychological wellbeing, future romantic relationships, and subsequent parenting of the next generation. The present study evaluates the cross-over effects of the PROSPER-delivered adolescent substance use prevention programming on young adult romantic relationship functioning through a long-term developmental cascade of adolescent skills and behaviors, along with subsequent family-of-origin functioning. Prospective, longitudinal, bivariate growth models were used to analyze the effects of the PROSPER-delivered interventions in a sample of 1008 youths living in rural and semi-rural communities in Iowa and Pennsylvania, starting in sixth grade (Age M = 11.8; 62% female) who were in a steady romantic relationship at the young adult assessment (Age M = 19.5). Findings indicated a cascading effect through which PROSPER promotes adolescent problemsolving skills during early-to-mid-adolescence; problem-solving skills were associated with better family functioning during mid-adolescence; and family functioning was associated with better romantic relationship quality, indicated by lower levels of relationship violence and more effective relationship problem-solving in young adulthood. PROSPER, which primarily targets adolescent substance misuse and conduct problem prevention, has lasting, collateral effects that benefit young adults in their romantic relationship functioning -which may have further downstream benefits for their own relationships and those of their children (i.e., intergenerational transmission effects). These findings add to the growing body of literature evidencing important cross-over effects of widely disseminated substance use prevention programs delivered during adolescence.