Background: School bullying is a widespread phenomenon across the world, which involves bystanders who take on various roles. Motivation to defend victims is important to investigate because it helps us devise better, evidence-based, anti-bullying interventions. Objective: We aimed to determine whether students' behavioral and emotional strengths and difficulties and student-teacher relationships were associated with different types of motivation to defend victims of bullying. The hypotheses were (1) emotional and behavioral difficulties will be associated with less autonomous and introjected motivation to defend and greater extrinsic motivation to defend and (2) close student-teacher relationships will be associated with greater autonomous motivation to defend, and less extrinsic motivation to defend. Method: Data were collected from 483 Swedish early adolescents who completed a survey in their classrooms. Results: Results showed that, among boys and girls, close student-teacher relationships were positively associated with autonomous motivation and negatively associated with extrinsic motivation to defend, while negative expectations concerning teachers were associated with all forms of motivation to defend. Emotional and behavioral difficulties were only associated with introjected motivation to defend among girls. Furthermore, extrinsic motivation to defend was associated with the interactions between individual differences in behavioral and emotional difficulties and negative expectations. Conclusions: Adolescents who are more occupied with wanting to have a better relationship with their teachers might be motivated to be involved in good social relationships with others. The results also indicate that closeness in student-teacher relationships is important for greater autonomous motivation to defend victims during bullying.