The current study investigated the mediating role of social achievement goals in the relation between classroom goal structures and academic engagement and social adjustment among 373 middle school students (52.8% female). Students' perceptions of classroom goal structures were measured in Fall; social achievement goals and academic and social outcomes were measured in Spring. Structural equation modeling analysis confirmed that social achievement goals partially mediated the effects of perceived classroom goal structures on academic and social outcomes. Perceived classroom mastery goal structure predicted social development goals positively but social demonstration-approach goals negatively. Perceived classroom performance goal structure positively predicted social demonstration-approach and avoid goals. Subsequently, social development goals were related to adaptive outcomes (e.g., academic engagement and social satisfaction) while social demonstration-approach goals predicted negative outcomes (e.g., disruptive behaviors and social worry). Interestingly, social demonstration-avoid goals predicted high social worry but low disruptive behaviors. Theoretical and practical implications are discussed. (Contains 2 tables and 2 figures.)