We examined whether the bystanders' behaviors in bullying situations influence vulnerable students' risk for victimization. The sample consisted of 6,980 primary school children from Grades 3–5, who were nested within 378 classrooms in 77 schools. These students filled out Internet-based questionnaires in their schools' computer labs. The results from multilevel models indicated that the associations between victimization and its two risk factors—social anxiety and peer rejection—were strongest in classrooms that were high in reinforcing bullying and low in defending the victims. This suggests that bystanders' behaviors in bullying situations moderate the effects of individual and interpersonal risk factors for victimization. Influencing these behaviors might be an effective way to protect vulnerable children from victimization.