Background <p>Prosocial behavior is an essential quality for medical students and reflects the development of future professional values. This study aims to investigate the roles of perceived social support and psychological capital in the prosocial behaviors of medical students while also testing the moderating effect of negative life events.</p> Methods <p>A cross-sectional survey was conducted from July to October 2023 among 1940 medical students in Jiangsu Province, using a questionnaire composed of the Prosocial Tendencies Measurement Scale, the Perceived Social Support Scale, the Psychological Capital Questionnaire, and the Adolescent Life Events Scale. SPSS 25.0 was used for the preliminary analysis, and M-plus 8.3 was used to examine the moderated mediation effect.</p> Results <p>Perceived social support (β = 0.248, 95% CI [0.178, 0.321]) and psychological capital (β = 0.414, 95% CI [0.334, 0.495]) positively predicted medical students’ prosocial behavior. Perceived social support positively predicted psychological capital (β = 0.458, 95% CI [0.401, 0.518]). Psychological capital mediated their relationship. The first half of the mediating pathway of “perceived social support → psychological capital → prosocial behavior” was moderated by negative life events. Specifically, due to the negative moderating effect of negative life events, the indirect impact of psychological capital on the relationship between perceived social support and prosocial behavior increased from 33.5% to 50.6%.</p> Conclusions <p>Perceived social support directly predicts medical students’ prosocial behavior and may do so indirectly via psychological capital. It’s a stronger positive predictor of psychological capital in medical students with lower negative life events. Findings help develop targeted mental health courses and interventions for medical students’ prosocial behaviors.</p>