Few studies have examined the process of the relationship between social support and bystander behavior, and previous research has primarily focused on individual- or group-level correlates of bystander behavior without a full understanding of these correlates and their interactions. To address this research gap, the present study examined whether social support and moral disengagement at the individual level, as well as collective efficacy at the classroom level, were associated with bystander behavior in school bullying situations. Questionnaires were administered to 1,310 elementary school students in Grades 4-6 (Mage = 10.97 ± 0.98 years) from 61 classes in the Zhejiang Province of China. Multilevel moderated mediation analyses revealed that higher levels of social support were directly related to more defender behavior and less reinforcer and outsider behavior, and social support was also related to bystander behavior through the indirect role of moral disengagement. In classrooms with higher levels of collective efficacy to stop peer bullying, students tended to show more defender behavior and less outsider behavior. Higher levels of classroom collective efficacy did not moderate the relationship between moral disengagement and defender behavior, although it did inhibit the reinforcer and outsider behaviors of those with high levels of moral disengagement. This study sheds light on the relationship between multiple factors and bystander behavior at both the individual and classroom levels and contributes to the understanding of school bullying in the context of peer group interactions. (PsycInfo Database Record (c) 2024 APA, all rights reserved).