Background: The development of prosocial behavior is a significant aspect of children's social well-being. From the perspective of positive psychology, subjective well-being (SWB) "in school" is likely a particularly relevant construct in elementary school-aged children's prosocial development. Research on the relation between SWB in school and prosocial behavior among elementary school-aged children is limited and the relation remains to be clarified using more stringent, longer-term longitudinal methodology and analyses. Objective: Our primary goal was to examine the longitudinal relations between SWB in school and prosocial behavior among elementary school-aged children in a longitudinal framework. Method: A total of 634 Chinese elementary school-aged children (baseline M[subscript age] = 9.01) participated on three measurement occasions across 2 years at 12-month intervals. Data on children's SWB in school and prosocial behavior was collected from participants' self-reports. Longitudinal measurement invariance was tested for the scales. A series of three-wave autoregressive cross-lagged models were used to examine whether SWB in school predicted prosocial behavior or vice versa, and whether the pattern of relations between them was stable over time. Results: Results revealed positive, bidirectional relations between SWB in school and prosocial behavior among elementary school-aged children, after controlling for gender, grade level, and parental education levels. This pattern of relations was stable over time. Conclusions: The relation between SWB in school and prosocial behavior is positive and bidirectional among elementary school-aged children. These results suggest the importance of comprehensive social-emotional interventions in promoting the positive development of elementary school-aged children.