The purpose of this study was to evaluate the efficacy of a brief, bystander bullying intervention on reducing internalizing symptoms among students (N = 65). Although witnessing bullying is associated with mental health risks, the majority of research on bystander interventions focuses on the impact of these programs on school-wide bullying reduction rather than improved emotional outcomes for those trained to intervene. Results indicated high school students trained in a brief, bystander bullying intervention reported greater decreases in internalizing symptoms from baseline to a 3-month follow-up compared to students in a control group. Further, gender moderated intervention effects such that differences in decreases in internalizing symptoms were significant for females only. Implications for school-based anti-bullying programs for high school students are discussed.