Ostracism involves two parties: the ostracizer and the ostracized. This research investigates its impact on prosocial behavior through two mediating pathways: self-dehumanization (from the ostracized individual's self-perception) and reduced in-group identification (from group-based perception). Study 1 established baseline correlations, with ostracism negatively predicting prosocial tendencies through both mediators. Study 2 (Recall Paradigm) found the effect of recalling in-group ostracism was fully mediated by self-dehumanization. Study 3, using the minimal group and cyberball paradigms, confirmed that exclusion reduces prosocial behavior through self-dehumanization and reduced in-group identification. Moreover, the mediating effect of in-group identification was significantly stronger for in-group versus out-group ostracism.This study demonstrates how ostracism affects prosocial behavior through the social perceptions of the self (the ostracized) and the group (the ostracizer), and reveals differences in these processes across group identities.