The aim of this study was to investigate whether various dimensions of student–teacher relationships were associated with different types of motivation to defend victims in bullying and to determine the association between these types of motivations and various bystander behaviors in bullying situations among early adolescents in Italy. Data were collected from 405 Italian adolescents who completed a survey in their classroom. Results showed that warm student–teacher relationships were positively associated with defending victims and autonomous motivation to defend victims. In contrast, conflictual student–teacher relationships were positively associated with passive bystanding and with extrinsic motivation to defend. Different forms of motivation to defend were found to be mediators between student–teacher relationship qualities and bystander behaviors in school bullying. Our findings suggest that teachers should build warm and caring student–teacher relationships to enhance students’ autonomous motivation to defend victims of bullying as well as their inclination to defend the victims in practice.