Children exposed to peer victimization are at increased risk for psychopathology. However, the physiological mechanisms linking peer victimization to child psychopathology and the potential gender differences in these links remain inadequately understood. The present study examined whether cortisol reactivity to acute stress mediated the associations between relational and physical victimization and internalizing and externalizing problems and whether these associations differed between boys and girls. A sample of 150 Chinese children (aged 9–13 years; Mage = 10.69 years; 51% boys) reported experiences of relational and physical victimization and participated in a standardized laboratory psychosocial stress task, during which six salivary cortisol samples were collected. Parents or primary caregivers reported their children’s internalizing and externalizing problems. Overall, neither physical nor relational victimization was associated with cortisol reactivity. However, when examined separately by gender, relational victimization was associated with blunted cortisol reactivity for boys but not for girls. Further, among boys but not girls, relational victimization was indirectly associated with internalizing and externalizing problems via blunted cortisol reactivity. Our findings suggest that blunted cortisol reactivity may serve as a physiological pathway linking peer victimization to psychopathology for boys but not for girls.