While many adolescents and young adults experiment with substances (e.g., alcohol, cigarette smoking, marijuana), recent research suggests that rural youth and young adults may be more at risk for substance use than their urban counterparts. This study was designed to examine the longitudinal relationships between rural adolescents' prosocial behaviors and substance use in young adulthood. Furthermore, we examined the potential mediating effects of adolescent substance use, academic investment, and delinquency. Rural youth (N = 531; 263 girls) were surveyed in grades 10-12 (Time 1; M age = 16.17; SD = 0.91) and again in early adulthood (Time 2). Measures of prosocial behaviors, substance use, academic investment, and deviant activities were assessed at Time 1. At Time 2, measures of marijuana use, cigarette smoking, and getting drunk were administered. Overall, the findings showed that rural adolescents who frequently exhibit prosocial behaviors are less likely to engage in substance use in young adulthood than those who exhibit relatively low levels of prosocial behaviors. These findings indicate that prosocial behaviors may have positive health consequences, establishing behavioral trajectories that lead to lower levels of risky health behaviors in adulthood in rural populations.