Many youth experience declines in psychosocial well-being during the transition from high school to postsecondary. Hypothesizing that extracurricular activity involvement in high school functions as a resource factor, the current study examines type and breadth as predictors of psychosocial well-being one year postsecondary. The sample (N = 4070) consisted of students from diverse ethnic-racial and socio-economic status backgrounds (30% Latinx; 60% had a parent without a college degree; 47% cisgender female). Eleventh grade involvement in sports was linked with lower loneliness, social anxiety, and depressive symptoms, and higher self-worth, whereas special interest clubs were associated with lower social anxiety and depressive symptoms. Examining breadth, one or two activities were linked with optimal psychosocial well-being. The results suggest that sports and special interest clubs, and up to two activity domains, are associated with optimal psychosocial well-being, providing recommendations for extracurricular programming and youth involvement.