Family members and friends can play an important role in adolescents’ prosocial behavior. To better understand the relation between support and prosocial behavior in adolescence, it’s important to conduct longitudinal studies that distinguish between within-dyad variance and between-dyad variance. The current study investigated longitudinal associations between adolescents’ prosocial behavior, autonomy support, and emotional support from family and friends across adolescence. Across six annual years, 497 Dutch adolescents (284 boys; mean age T1 = 13.03 years, SDage = 0.46), fathers, mothers, siblings, and friends reported on their prosocial behavior. Adolescents also reported on perceived autonomy and emotional support. Between-dyads almost all associations of support and prosocial behavior of family members and friends with adolescents’ prosocial behavior were significant, with higher levels of adolescents’ prosocial behavior being associated with higher levels of prosocial behavior and support from fathers, mothers and friends. Within-dyads, several concurrent associations were significant, but within-dyads links between prosocial behavior and autonomy support are particularly driven by adolescent-mother or adolescent-sibling effects. This study highlights processes that occurred either at the between-dyad level or at the within-dyad level, but that varied per relationship type and that adolescents are the main catalysts in within-dyads changes in prosocial behavior and support. Preregistration: This study was preregistered on 20 January 2020 at https://osf.io/vxkm3/?view_only=dca87fd1585c444ba5cd5a00c22280ae
