This study investigates the role of a social context variable, perceived peer norms, in the relationship between media violence exposure and adolescents’ aggressive behavior. This was informed by a need to better understand whether, how, and for whom, media violence exposure may affect aggression. Three hypotheses were tested with peer norms as moderator, as mediator, and as both moderator and mediator in the relationship between media violence and aggression. A two-wave longitudinal survey measured media violence exposure, perceived descriptive and injunctive norms, and aggressive behavior among 943 adolescents (aged 10–14, 50.4% girls). Results provided support only for the moderated-mediation model. The indirect effect of media violence on aggression via perceived peer approval of aggression (i.e., injunctive norms) was moderated by perceived prevalence of peer aggression (i.e., descriptive norms). Specifically, media violence indirectly increased aggressive behavior for adolescents who perceived more peer aggression, but decreased aggression for adolescents who perceived less peer aggression. Implications for future research into media violence effects are discussed.