Firearm injury is a significant public health concern among youth living in the United States. Youth with exposure to violence (ETV) are more susceptible to carrying and using a firearm. Few researchers, however, have examined psychological mechanisms undergirding the association between ETV and firearm aggression. Retaliatory attitudes have been discussed as a potential mediator linking ETV with firearm aggression. Moreover, organized activity participation may disrupt direct and indirect pathways connecting ETV to firearm aggression. We tested: (a) the mediating role of retaliatory attitudes in the ETV-firearm aggression link, and (b) the moderating role of organized activity participation among 570 youth with past year illicit drug use and seeking emerging department care in an urban emergency department (ages 14-24; 58.8% males). Using multigroup path analysis, ETV indirectly influenced firearm aggression through retaliatory attitudes for youth not involved organized activities. Organized activities also buffered the association between retaliatory attitudes (mediator) and firearm aggression (outcome). Organized activities may, therefore, prevent firearm aggression by reducing retaliatory attitudes among youth contending with ETV.