This study investigated the impact of social stress on symptoms of psychopathology at the entry into adolescence (111 girls, Mage = 11.84, SD = 0.77). We examined whether peer stress and pubertal timing were associated with internalizing distress and aggression, and whether responses to stress and cortisol reactivity mediated or moderated these associations. Cortisol samples were collected from saliva samples during in-home visits, and the YSR was used to assess psychopathology. Interestingly, pubertal timing demonstrated a trend association with cortisol. Responses to stress mediated the association between social stress and symptoms of internalizing distress and aggression. Specifically, early maturers and girls with higher levels of peer stress exhibited more problematic responses to stress, in turn demonstrating higher levels of internalizing distress and aggression. Significant moderation effects also emerged. For example, early maturers who experienced higher levels of emotional/cognitive numbing in response to peer stress were at greater risk for aggression. Findings identify coping strategies that may be used in evidence-based programming to help girls transition more successfully into adolescence will be discussed.