Friends’ moral characteristics such as their moral reasoning represent an important social contextual factor for children’s behavioral socialization. Guided by this assumption, we compared the effects of children’s and friends’ moral reasoning on their aggressive behavior in a low-risk sample of elementary school children. Peer nominations and teacher reports were used to assess children’s aggressive behavior and friendships. During individual interviews, moral reasoning was measured by justifications following moral judgments and moral emotion attributions. Results revealed that, compared to individuals’ moral reasoning, friends’ moral reasoning was more consistently related to children’s aggressive behavior. Moreover, friends’ aggressive behavior mediated the relationship between friends’ moral reasoning and children’s aggressive behavior. The findings provide evidence for the important role that friends’ moral development plays in children’s behavioral socialization, and highlight the need for integrated, systematic approaches to moral development and friendship relations.