Past research suggests that parents’ familism values play a positive role in Latinx American youth’s prosocial tendencies. However, little is known about how individual differences in youth’s neural development may contribute to this developmental process. Therefore, using two-wave longitudinal data of 1916 early adolescents (mean age = 9.90 years; 50% girls) and their parents (mean age = 38.43 years; 90% mothers) from the Adolescent Brain Cognitive Development study, this pre-registered study took a biopsychosocial approach to examine the moderating role of youth’s neural reward sensitivity in the link between parents’ familism values and youth’s prosocial behaviors. Results showed that parents’ familism values were associated with increased prosocial behaviors among youth two years later, controlling for baseline prosocial behaviors and demographic covariates. Notably, parents’ familism values played a larger role in promoting youth’s prosocial behaviors among youth who showed lower ventral striatum activation during reward anticipation. Moreover, such association between parents’ familism values and youth’s later prosocial behaviors was stronger among youth who showed lower levels of prosocial behaviors initially. Taken together, the findings highlight individual differences in neurobiological development and baseline prosocial behaviors as markers of sensitivity to cultural environments with regard to Latinx American youth’s prosocial development.