This study examined the association between ethnicity-related context variables and the prosocial behavior of early adolescents in ethnically/racially diverse schools. Specifically, youths’ perceptions of greater representation of same-ethnic peers at school, school support for ethnic diversity, and engagement in and valuing cross-ethnic contact across varying levels of school ethnic/racial diversity were examined in relation to their prosocial behavior toward peers. Multilevel analyses indicated that prosociability in a sample of 2,369 sixth-grade students in 20 ethnically/racially diverse middle schools was associated with student engagement in and valuing of cross-ethnic contact in circumstances of relatively lower and average school ethnic/racial diversity but not higher diversity. In addition, prosocial behavior was marginally associated with greater representation of same-ethnic peers at school but not associated with school support for ethnic diversity. Findings offer new evidence of the psychological significance of experiences of ethnic similarity and diversity for early adolescents’ prosocial behavior.