Objective <p>In the context of mobile internet use, adolescents’ online engagement is increasingly intertwined with offline socialization. Although previous studies have revealed a negative association between internet addiction and social adaptation, they have often examined peer relationships and self-identity as separate mediators, lacking systematic validation of their sequential role. Guided by the Bioecological Model, this study examined the chain mediation of peer relationships and self-identity between internet addiction and adolescents’ social adaptation, and further compared gender differences in the association between peer relationships and self-identity.</p> Methods <p>We conducted a cross-sectional study among 1,080 students from multiple middle schools in China. Standardized scales were used to assess internet addiction, peer relationships, self-identity, and social adaptation. Structural equation modeling and multigroup analysis were employed for statistical testing.</p> Results <p>Internet addiction was negatively associated with social adaptation. Peer relationships and self-identity jointly mediated this relationship in a sequential manner. Multigroup analysis indicated gender differences in the positive association between peer relationships and self-identity, showing a stronger effect among male students.</p> Conclusion <p>Within the Bioecological Model, this study jointly rather than independently validated the mediating roles of peer relationships and self-identity, providing systematic evidence for the association between internet addiction and social adaptation. In practice, interventions should prioritize strengthening peer support and fostering its connection with self-identity and social adaptation, while paying appropriate attention to male adolescents.</p>