This study demonstrates the effectiveness of the KiVa antibullying program using a large sample of 8,237 youth from Grades 4–6 (10–12 years). Altogether, 78 schools were randomly assigned to intervention (39 schools, 4,207 students) and control conditions (39 schools, 4,030 students). Multilevel regression analyses revealed that after 9 months of implementation, the intervention had consistent beneficial effects on 7 of the 11 dependent variables, including self- and peer-reported victimization and self-reported bullying. The results indicate that the KiVa program is effective in reducing school bullying and victimization in Grades 4–6. Despite some evidence against school-based interventions, the results suggest that well-conceived school-based programs can reduce victimization.