Rhythmic activities such as joint music-making and synchronous moving are known to produce prosocial effects in both adults and children, but the underlying mechanism remains unclear. One possible mechanism is that the time-locked, fine-grained coordination characteristic of rhythmic activities plays a key role in producing powerful prosocial effects. The present study investigated how coordination in a joint music-making task would influence kindergarteners' prosociality toward both coperformers and unaffiliated strangers. The study involved 138 Chinese children (72 girls, M = 5 years and 6 months, range = 5.0 to 6.0 years) from urban middle-class families. Participants were paired and instructed to play percussion instruments in alternation accompanying a song. In the fine-grained coordination condition, the dyad alternated every measure, resulting in a moment-to-moment coordinative experience; in the coarse-grained coordination condition where the coordination was sparser, the dyad alternated every eight measures. Children in the fine-grained coordination condition were subsequently more willing to help their partner complete a block-assembly task and more generous in sharing stickers with unknown children in a dictator game, compared with children in the coarse-grained coordination condition. These findings demonstrate that fine-grained coordination in rhythmic activities increases prosociality above and beyond having a shared goal of coperforming, supporting that coordination is an integral part of the prosocial mechanism. The prosocial effects of joint rhythmic activities generalized beyond the coperformers to anonymous strangers, indicating that the role of coordination may change from directing specific bonding in infancy to encouraging general prosociality from early childhood and onward.