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.