Introduction: This study examined the longitudinal associations between environmental adversity (defined in terms of exposure to violence in the neighborhood, school, and media), complex trauma (operationalized as experiences of abuse and neglect), and adolescents’ internalizing and externalizing symptoms. Methods: Using a cross-lagged panel research design, we investigated the moderating role of peer support in these relationships in a sample of 644 adolescents from a severely disadvantaged district of Lima, Peru, who were followed up in a 1-year prospective study. Results and conclusions: We found significant unidirectional dynamic relations, where both types of adversity were associated with higher levels of internalizing and externalizing symptoms. Peer support significantly moderated this effect, but only for complex trauma, in that higher levels of peer support were associated with a decreased impact of complex trauma on internalizing and externalizing symptoms. These findings highlight the importance of social relations and the quality of peer relations in particular as factors that may mitigate the risk of early exposure to trauma.