This study examined processes by which extracurricular participation is linked with positive ethnic intergroup attitudes in multiethnic middle schools in California. Specifically, the mediating roles of activity‐related cross‐ethnic friendships and social identities including alliances with multiple groups were examined in a sample including African American or Black, East or South‐East Asian, White, and Latino youth (N = 1,446; Mage = 11.60 in sixth grade). Results of multilevel modeling suggested that in addition to activity‐related cross‐ethnic friendships, complex social identities mediated the association between availability of cross‐ethnic peers in activities and ethnic intergroup attitudes. Results are discussed in terms of how activities can be structured to promote cross‐ethnic relationships and complex social identities, as well as positive ethnic intergroup attitudes.