Canada's high school graduation rates are still low when compared to other members of the OECD. Previous studies have found academic involvement is associated with positive trajectories toward graduation, that social support promotes student engagement, and that school belonging could mediate this relationship. Still, little is known about the specificity of such mediation, especially in Québec. Therefore, this study examined the role of belonging as mediator of the relationship between social support and academic involvement. Participants (N = 238) were high-school students from the Greater Montréal Area. All variables were measured by the School-Climate Questionnaire. Results from hierarchical multiple regressions indicated parental support had a direct relationship, whereas peer and teacher support had a mediated relationship by school belonging with academic involvement. Results highlight the critical role of school belonging in promoting academic involvement in relation to social support.