Self-definition becomes a central task during early adolescence, as youth identify with multiple social groups. Focusing on ethnic identification and identification with other salient social ingroups (e.g., those based on extracurricular activities), we examined predictors of perceived ingroup membership overlap among ethnically diverse adolescents (n = 1264; M (age) = 12.51; 53 % female). Social ingroup overlap remained relatively stable, but decreased, across the seventh to eighth grade. The number of cross-ethnic classmates in seventh grade predicted lower overlap in the eighth grade, and cross-ethnic friendships by eighth grade mediated this association. Findings underscore the opportunities provided by multiethnic middle schools for youth to connect and befriend one another across ethnic lines, as well as to foster divergent social identities that are associated with positive intergroup attitudes.