Prosociality has long been emphasized by collectivist societies that prioritize group interdependence. Therefore, understanding what contributes to prosocial tendencies within the collectivist cultural context is of theoretical and practical interest. Previous Western studies have suggested a vital role of grandparents in young people’s prosocial tendencies. However, whether this association holds in non-Western cultural contexts and how grandparents are related to prosocial tendencies in grandchildren remains unclear. Therefore, the current study explored empathy (cognitive/affective) as a potential mediator to explain the relationship between grandparental involvement and grandchildren’s prosocial tendencies from a Chinese adult grandchildren’s perspective. A sample of 321 young adults aged 18–30 years participated in this study. Using structural equation modeling, we found that perceived grandparental involvement was only significantly and directly related to increased cognitive empathy, whereas both cognitive and affective empathy were significantly and directly associated with heightened prosocial tendencies. Moreover, mediation results showed that perceived grandparental involvement indirectly contributed to prosocial tendencies in young adult grandchildren solely through the mediating role of cognitive empathy. Echoing the social learning theory, this study suggested that interacting with caring and supportive grandparents can connect with the likelihood of individuals developing abilities to take others’ perspectives, which, in turn, motivates them to exhibit greater tendencies toward prosocial behaviors. These findings shed light on the salience of grandparents in young people’s moral development.