The response patterns of youth who witness bullying have potential to influence future occurrence and persistence of bullying behaviors. Prevention efforts have included an increased focus on bystander interventions; however, the majority of adolescents rarely intervene in bullying situations. When they do, they often display of mixed pattern of behaviors. To better understand factors related to adolescent responses to witnessing bullying, the current study drew upon school climate survey data reported by 57,314 middle and high school students. Logistic regression analyses demonstrated higher odds of bully assisting behavior among those with higher externalizing symptoms and higher odds of defending the targets among those with elevated internalizing symptoms. Prior victimization was associated with higher odds of all bystander-behavior response types. These findings highlight the salient role of youth's prior victimization and mental health in youth responses to witnessing bullying. Implications for bystander-focused bullying prevention efforts are considered.