Guided by the social-cognitive theory and self-determination theory, this study examined whether moral disengagement is indirectly associated with pro-bullying, passive bystanding, and defending, mediated by autonomous motivation, introjected motivation, extrinsic motivation, and amotivation to defend victims of bullying among early adolescents. Participants were 901 upper elementary students from 43 school classes at 15 public schools in Sweden who completed a questionnaire in their classrooms. The results showed that students who were less inclined to morally disengage in peer bullying tended to be more autonomously motivated to take the victim's side, which in turn was associated with greater defending and fewer pro-bullying behaviors. Introjected motivation to defend negatively mediated the association between moral disengagement and defending, and positively mediated moral disengagement's associations with passive bystanding and pro-bullying behavior. Extrinsic motivation to defend mediated moral disengagement's associations with passive bystanding and pro-bullying behavior. Finally, students who were more prone to morally disengage in peer bullying tended to be more amotivated to take the victim's side, which in turn was associated with greater pro-bullying behavior and less defending.