This study examined pathways from prosocial behaviour in the beginning of the school year to emotional health and academic achievement in the end of the year, taking into account the mediating role of peer acceptance. Participants were 734 grade 4 to 7 students in public elementary schools (51% female) in Western Canada. As expected, pathway analyses taking into account the clustered structure of the data indicated that self-reported prosocial behaviour in the beginning of the school year was significantly related higher levels of self-reported optimism, lower levels of depressive symptoms, and better grades in the end of the year; the association was mediated by peer-reported peer acceptance in the classroom. Findings support the role of peer acceptance as an underlying mechanism involved in the association between prosocial behaviour and positive developmental health outcomes in early adolescence. Findings are also practically relevant as they inform school-based social-emotional competence promotion through interventions.