Given theoretical and empirical support for the importance of peer play within the preschool classroom to early learning, the present study tested the hypothesis that associations between teacher-reported problem behavior and academic skills were mediated by difficulties in peer play (disruptive and disconnected play), for a representative sample (N = 507 children across 46 classrooms) enrolled in a large urban Head Start program. The study also tested whether mediation was moderated by child sex (e.g., whether the mechanism through which problem behavior was associated with learning differed for boys and girls) controlling for the multilevel nature of these data. Findings from structural equation models provided evidence for partial mediation, whereby associations between internalizing behavior and academic skills were partially explained through their effects on disconnected play; however, mediation was present for girls only. Implications for theory, research, and practice are discussed as they support low-income children's social and academic development through the context of peer play. (author abstract)