This study evaluated the relationship among use defending behaviors, gender, and self-esteem among students trained in a brief, bystander bullying intervention (N = 93). Students were taught four specific strategies to use to defend targets of bullying. We used hierarchical regression analyses to test a moderator model in which we hypothesized females would report using defending behaviors more frequently than males post-training and that baseline self-esteem would moderate this relationship. Findings partially supported our hypotheses. Specifically, for “Turning it Over” (i.e., reporting bullying to an adult) and “Coaching Compassion” (i.e., gently confronting the perpetrator to communicate their behavior is not acceptable), the gender x self-esteem interaction was significant, suggesting that females with high self-esteem were most likely to use these defending behaviors post-training. In contrast, for “Stealing the Show” (i.e., using their sense of humor to distract the peer audience’s attention away from the target), high self-esteem was positively associated with frequency of using this defending behavior for both males and females. For “Accompanying Others” (i.e., reaching out to the target to offer support), females used this strategy more frequently than males, and self-esteem was not a significant moderator. Findings highlight the importance of gender and self-esteem as significant factors that influence whether or not bystanders defend targets of bullying post-training. Implications for bystander training in school-based bullying prevention programs are discussed.