This longitudinal study examines the mediating mechanisms by which the KiVa antibullying program, based on the Participant Role approach, reduces bullying and victimization among elementary school students. Both student-level mechanisms leading to reduced perpetration of bullying and classroom-level mechanisms leading to reductions in bullying and victimization are considered. Analyses are based on a sample of 7,491 students (49.5% boys) nested within 421 classrooms within 77 schools. At the beginning of program implementation, the children were in Grades 4, 5, and 6 (mean age 11.3 years). Multilevel structural equation modeling was used to analyze whether changes in the hypothesized mediators accounted for later reductions in the outcomes. At the student level, antibullying attitudes and perceptions regarding peers' defending behaviors and teacher attitudes toward bullying mediated the effects of KiVa on self-reported bullying perpetration. The effects on peer-reported bullying were only mediated by antibullying attitudes. At the classroom level, the program effects on both self- and peer-reported bullying were mediated by students' collective perceptions of teacher attitudes toward bullying. Also, perceived reinforcing behaviors predicted bullying but did not emerge as a significant mediator. Finally, bullying mediated the effects of the classroom-level factors on victimization. These findings enhance knowledge of the psychosocial developmental processes contributing to bullying and victimization and shed light on the key mechanisms by which school bullying can successfully be counteracted.