This study investigated contextual antecedents (i.e., cross‐ethnic peers and friends) and correlates (i.e., intergroup attitudes) of social identity complexity in seventh grade. Social identity complexity refers to the perceived overlap among social groups with which youth identify. Identifying mostly with out‐of‐school sports, religious affiliations, and peer crowds, the ethnically diverse sample (N = 622; Mage in seventh grade = 12.56) showed moderately high complexity. Social identity complexity mediated the link between cross‐ethnic friendships and ethnic intergroup attitudes, but only when adolescents had a high proportion of cross‐ethnic peers at school. Results are discussed in terms of how school diversity can promote complex social identities and positive intergroup attitudes.