To supplement research on adolescent social identities, the current study examined how social identity complexity relates to ethnic inter‐group attitudes in a young adolescent sample (N = 97; age range = 12–14 years). Social identity complexity refers to the perceived overlap of groups with which youth align themselves. Descriptive analyses revealed that the most prevalent social groups were based on out‐of‐school sports and in‐school extracurricular activities. On average, participants reported a moderate degree of overlap among their social in‐groups. Results of regression analyses showed that high social identity complexity relates to positive inter‐group attitudes, both cross‐sectionally in seventh grade and longitudinally across eighth grade. These findings suggest that high social identity complexity may have implications for the ways in which school‐based activity groups are structured to promote inter‐group attitudes.