Introduction: Students’ peer relatedness is vital to their academic achievement and engagement. However, little is known about the mechanisms that can explain such a link in health professions education, especially in interprofessional education (IPE), where interprofessional socialization is promoted. To address the research gap in understanding the social dynamics embedded within IPE and their impact on crucial motivational outcomes, this study examines how peer relatedness (belonging) mediates the link between motivation (both intrinsic and extrinsic) and engagement in IPE. Methods: Data from 841 students enrolled in IPE from Medicine, Nursing, Social Work, Chinese Medicine, Pharmacy, Speech and Hearing Sciences, Clinical Psychology, and Food and Nutritional Science from a university in Hong Kong were used in this study. Intrinsic and extrinsic motivation were collected at baseline, while peer relatedness need satisfaction and engagement and disaffection in IPE were assessed after four weeks. We used a fully latent structural equation model to examine whether peer relatedness mediated the link between motivation and engagement in IPE. Results: Our results indicate that intrinsic motivation at baseline significantly increases engagement and reduces disaffection four weeks later, while extrinsic motivation shows the opposite effect. Crucially, the adaptive role of motivation to engagement was significantly mediated by relatedness from IPE teams. Specifically, the effect of students’ motivation on their engagement/disaffection in IPE can be partially explained by their relatedness within IPE teams. Conclusion: This study underscores the importance of peer relatedness in IPE, highlighting its role in harnessing student motivation to foster student engagement. The findings contribute to a deeper understanding of the psychosocial mechanisms in IPE and highlight the value of collaborative learning environments in fulfilling students’ need for relatedness, thereby fostering