The goal of this study was to examine how individual differences in physiological arousal and reactivity moderated the relation between peer victimization and reactive and proactive aggression. Participants were 58 adolescents (61.2% boys; 54.9% African American) in the age range of 12--15. Participants self-reported peer victimization, reactive aggression, and proactive aggression. Cortisol and respiratory sinus arrhythmia (RSA) were measured from participants before and during an online game in which they were socially rejected by unfamiliar peers. Results indicated that the relation between peer victimization with reactive aggression was significant and positive at low levels of resting RSA and when RSA withdrawal after rejection was high. The association between peer victimization with reactive and proactive aggression was also significant and positive at high levels of anticipatory cortisol. Findings provide further insight into the moderating role that physiological processes may have in understanding individual differences to peer adversity.