Although a bulk of literature shows that perceived social support (PSS) influences academic achievement, the mechanisms through which this effect operates received little empirical attention. The present study examined the multiple mediational effects of motivational beliefs (competence beliefs and subjective value) and emotions (anxiety and enjoyment) that may account for the empirical link between PSS (from parents, peers and teachers) and mathematics achievement. The participants of the study were 238 grade 7 students (average age = 13.2 years, girls = 54%, predominantly native Dutch middle class socioeconomic status). A bootstrap analysis (a relatively new technique for testing multiple mediation) revealed that the motivational beliefs and the emotions, jointly, partially mediated the effect of PSS on achievement. The proportion of the effects mediated, however, varied across the support sources from 55% to 75%. The findings lend support to the theoretical assumptions in the literature that supportive social relationships influence achievement through motivational and affective pathways.