In bullying situations, adolescent bystanders may help bullied others, just stand by, or join in the antisocial behavior. Current studies have yet to fully examine the moral and social factors motivating these varied responses to bullying encounters. Extending from pertinent developmental theories, the present study investigated the relative contributions of moral identity (i.e., viewing moral qualities as central to the self), moral judgment, and social self-efficacy to adolescents' bystander behaviors vis-à-vis bullies. Also investigated were the interactions among these variables. Three hundred and thirty-seven adolescents (M age = 13 years, 56.1% female) who self-identified as Caucasian (90.2%), Hispanic-American (2.1%), Asian–American (0.9%), African–American (3.9%), or Other/Unknown (2.9%) participated in the study. Students completed questionnaires assessing moral identity, moral judgment, social self-efficacy, and how they would respond if they observed a peer being bullied. Moral identity predicted more prosocial action, particularly for adolescents high in social self-efficacy. Moral identity related positively to moral judgment, and both predicted less antisocial (joining in) behavior. Interestingly, moral judgment maturity primarily diminished antisocial behavior when moral identity was relatively low. Social self-efficacy predicted less passive bystanding. Overall, moral identity strongly relates to defending behavior, and—as does moral judgment maturity—predicts less antisocial behavior among bystanders.