Connections between prosociality and antisocial behaviors have been recognized; however, little research has studied their developmental links longitudinally. This is important to illuminate during early adolescence as a sensitive period for social development in which prosociality could protect against the development of later antisocial behaviors. This study investigates the within-person developmental links between prosociality and antisocial behaviors, as well as a potential mediating role of peer relationships, across ages 11, 13, and 15 (N = 1526; 51% male) using random-intercept cross-lagged panel models. Results indicated that neither self-reported nor teacher-reported prosociality was associated with reduced aggressive behaviors but suggested a direct protective ('promotive') effect of teacher-reported prosociality on bullying perpetration. These findings suggest that promoting prosociality in early adolescence may help reduce some antisocial behaviors over early to mid-adolescent development. Improving prosociality could be explored as a target in intervention approaches such as school-based anti-bullying interventions.