Interpersonal rejection has been found to impair people’s prosocial actions. However, this relation may change as a function of theoretically relevant personality traits. The present study examined the potential moderating roles of trait mindfulness in the longitudinal relations between peer rejection and prosocial behaviors. At two time points with an interval of eight months, 654 adolescents nominated children in their class who have been rejected by peers and children who have enacted prosocial behaviors toward classmates and then completed the mindfulness questionnaire. Cross-lagged model results indicated that peer rejection could negatively predict prosocial behaviors in both high and low mindfulness groups of children while the reverse predictive relation from prosociality to peer rejection was not significant. More prominent, trait mindfulness might buffer the negative contribution of peer rejection to prosocial behaviors. Within a more ecologically valid context with a longitudinal design, our study contributed to a better understanding of the conditions under which interpersonal rejection may undermine prosocial behaviors.