BACKGROUND : Although peers' acceptance of aggression is a major risk for the development of aggressive behavior, not all individuals who are situated within an aggression approving peer group engage in aggression. The present longitudinal study examined prosocial behavior as a moderator of the link between peers' acceptance of aggression and individual physical aggression. METHODS : The study used two waves of data of a large longitudinal study conducted in Germany. Self-reports of 1663 male and female children and adolescents aged between 10 and 20 years were used as measures for physical aggression, peers' acceptance of aggression, and prosocial behavior. RESULTS : Latent moderated structural equation modeling revealed significant main effects of peers' acceptance of aggression at T1 and prosocial behavior at T1 on aggressive behavior at T2. Most importantly, a significant interaction between both constructs indicated that the increase in individual aggression with peers' acceptance of aggression depended on participants' level of prosocial behavior. Applying the Johnson-Neyman technique, peers' acceptance of aggression was found to promote aggression only for participants with low levels of prosocial behavior, but not for moderately or highly prosocial individuals. CONCLUSIONS : The findings suggest that prosocial behavior has the propensity to attenuate the negative effect of peers' acceptance of aggression in the etiology of physical aggression in childhood and adolescence.