Universal and selective preventive interventions targeting youth behavioral problems have shown crossover effects on suicide risk, the second leading cause of death among youth. However, the mechanisms that explain this long-term unanticipated benefit are understudied and unclear. The current study examines the crossover effects of PROSPER, a community-university partnership model for delivering interventions for the prevention of adolescent substance misuse. We examine whether intervention effects on developmental trajectories of parent-child relationship quality and school belongingness explain the putative crossover effects. The analytical sample was 1,974 youth who participated in a randomized controlled trial of PROSPER in the 6th grade and completed an age 19 follow-up assessment. Participants completed annual assessments of parent-child relationship quality and school belongingness from the 6th to 12th grades, and reported on suicidal thoughts during the young adulthood assessment. Our developmental cascade model showed that PROSPER reduced the magnitude of declines in youths' reports of school belongingness from the 6th to 12th grade. In turn, youth who reported less decline in school belongingness reported fewer depression symptoms and suicidal thoughts during young adulthood. Study findings highlight the role of decline in school belongingness as a factor that contributes to the effects of universal prevention programs on youth suicide risk.