Extracurricular activities represent a key setting for prevention and promoting positive youth development. However, to date, a crucial aspect of activity participation--activity breadth (participation in a variety of extracurricular settings)--has been largely overlooked as a resource for bolstering adolescents' long-term adjustment, especially the development of self-concept. To examine the long-term psychosocial effects of involvement in multiple extracurricular settings, this study modeled latent trajectories of general, social, and academic self-concept and intensity and breadth of participation across 5 years (Grades 8-12) for 1,146 Australian youth (55% female; age[subscript wave 1] 12-14). We investigated multivariate change in self-concept and breadth of participation, while concurrently modeling intensity of participation. Self-concept and breadth followed a quadratic trajectory, declining across the early to-middle high school years and increasing during the final high school years. Intensity was also quadratic, but increased early on, followed by steep declines. Notably (and controlling for intensity), wider breadth of participation at the transition to high school predicted less-steep declines in general and academic self-concept across the early to-middle high school years. Findings support the potential for breadth of participation as a promising avenue for stimulating adolescents' adjustment by buffering against early declines in their views of self.