Prosocial peer relationships, such as defending against victimization, are beneficial for integration. Using the concept of multiple categorization, this study considers the extent to which similarity in gender, being in the same classroom, and similarity in network position regarding bullying or victimization contributes to the formation of cross-ethnic defending relationships among children. Longitudinal social network models were applied to complete school-level networks of 1,325 children in eight multi-ethnic elementary schools. Although same-ethnic peers were more likely to defend each other than cross-ethnic peers, similarity in gender, being in the same classroom, and similarity in network position in bullying fostered cross-ethnic defending. Moreover, being in the same classroom increased the likelihood of cross-ethnic defending even more than it did same-ethnic defending. A better understanding of how multiple categorization contributes to positive relationships between peers of different ethnic backgrounds may help to promote interethnic integration in multi-ethnic classrooms.