This study examined the role of children's trust beliefs and trustworthiness in the development of prosocial behavior using data from four waves of a longitudinal study in a large, ethnically-diverse sample of children in Switzerland (mean age = 8.11 years at Time 1, N = 1,028). Prosocial behavior directed towards peers was measured at all assessment points by teacher reports. Children's trust beliefs and their trustworthiness with peers were assessed and calculated by a social relations analysis at the first assessment point using children's reports of the extent to which classmates kept promises. In addition, teacher reports of children's trustworthiness were assessed at all four assessment points. Latent growth curve modeling yielded a decrease in prosocial behavior over time. Peer- and teacher-reported trustworthiness predicted higher initial levels of prosocial behavior, and peer-reported trustworthiness predicted less steep decreases in prosocial behavior over time. Autoregressive cross-lagged analysis also revealed bidirectional longitudinal associations between teacher-reported trustworthiness and prosocial behavior. We discuss the implications of the findings for research on the role of trust in the development of children's prosocial behavior.