Cyberbullying among school-aged children has received increased attention in recent literature. However, no empirical evidence currently exists on whether existing school-based antibullying programs are effective in targeting the unique aspects of cyberbullying. To address this important gap, the present study investigates the unique effects of the KiVa Antibullying Program on the frequency of cyberbullying and cybervictimization among elementary and middle school youth. Using data from a group randomized controlled trial, multilevel ordinal regression analyses were used to examine differences in the frequencies of cyberbullying and cybervictimization between intervention (N = 9,914) and control students (N = 8,498). The effects of age and gender on frequencies of cyber behaviors were also assessed across conditions. Results revealed a significant intervention effect on the frequency of cybervictimization; KiVa students reported lower frequencies of cybervictimization at posttest than students in a control condition. The effect of condition on the perpetration of cyberbullying was moderated by age. When student age was below the sample mean, KiVa students reported lower frequencies of cyberbullying than students in the control condition. We also found evidence of classroom level variation in cyberbullying and cybervictimization, suggesting cyberbullying is in part a classroom-level phenomenon. KiVa appears to be an efficacious program to address cyber forms of bullying and victimization. We discuss several unique aspects of KiVa that may account for the significant intervention effects. Results suggest that KiVa is an intervention option for schools concerned with reducing cyberbullying behavior and its deleterious effects on children's adjustment.