The association between prosocial behavior and adolescents' social class--defined as one's parents' position in society assessed in terms of education, income, and occupation status--is far from conclusive and the moderators of the link have not been fully researched. Using data from the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development (NICHD SECCYD) (N = 1,058), this study investigated the association between social class and 5th graders' prosocial behavior, as well as the moderating roles of maternal sensitivity, positive family emotional expressiveness, teacher-student relationship, and peer support. The current research found that social class was positively associated with adolescents' prosocial behavior, and the positive association between social class and prosocial behavior was less pronounced for adolescents with higher levels of maternal sensitivity, positive family emotional expressiveness, teacher-student relationship, and peer support than those with lower levels of maternal sensitivity, positive family emotional expressiveness, teacher-student relationship, and peer support. The present findings yield evidence on the negative influence of low social class on adolescents' prosocial behavior, but this effect would be buffered by supportive and healthy relationships in family and school contexts.