Social support is a crucial factor in the academic engagement of doctoral students, which is vital to their overall success. While past studies have mostly focused on the support from doctoral supervisors, support from other significant groups, including institutions, peers and families, has been largely neglected, and even no study has investigated their contributions to doctoral students' development. Drawing from the job demands-resources model, this study investigated the contributions of different sources (institutions, supervisors, peers and families) of social support to doctoral students' academic engagement. It further examined the mediating role of grit within these relationships. A sample of 472 doctoral students across various disciplines from 10 universities in mainland China responded to an online survey. Results showed that institutional support and supervisory support positively predicted doctoral students' academic engagement, while peer support and family support did not significantly predict academic engagement. Furthermore, grit was found to mediate the relationship between support from institutions and supervisors and academic engagement. Practical implications for higher education institutions, doctoral supervisors and programs are discussed.