Academic identity is a key factor that facilitates learning. Many factors influence academic identity, and peers exert crucial influence that has been broadened by social media. Therefore, this study focuses on the possible impact of this broadened scope of peer relationships on adolescents' development of academic identity. A retrospective analysis was adopted to investigate how recent high school graduates perceived online social capital, peer relationships and academic identity during high school life. A total of 1,286 Chinese high school graduates (510 males and 776 females) participated in the survey. Correlation, mediated regression and moderated multiple regressions were applied to data analysis. Results confirmed that the surveyed Chinese adolescents frequently used social media, and they accessed such platforms mostly at home. These students perceived more emotional than substantive support from social media, and their use of these platforms affected the observed academic identity. This effect was reflected in the positive association between online social capital and academic identity and the partial mediating role of peer relationships on this association. Meanwhile, the gender and region variables moderated the association between online social capital and academic identity. Finally, the implications of these results for research and practice were discussed.