Cyberbullying is a major public health problem associated with serious mental, social, and academic consequences for young people. To date, few programs addressing cyberbullying have been developed and empirically tested. The Cyber Friendly Schools (CFS) group-randomized controlled trial measured the longitudinal impact of a whole-school online cyberbullying prevention and intervention program, developed in partnership with young people. Non-government secondary schools in Perth, Western Australia, (N = 35; 3,000+ students) were randomized to an intervention (n= 19) or usual practice control group (n= 16 schools). Students completed online questionnaires in 2010, 2011, and at 1-year follow-up in 2012, measuring their cyberbullying experiences during the previous school term. The intervention group received the program in Grades 8 and 9 (aged 13–14 years). Program effects were tested using two-part growth models. The program was associated with significantly greater declines in the odds of involvement in cyber-victimization and perpetration from pre- to the first post-test, but no other differences were evident between the study conditions. However, teachers implemented only one third of the program content. More work is needed to build teacher capacity and self-efficacy to effectively implement cyberbullying programs. Whole-school cyberbullying interventions implemented in conjunction with other bullying prevention programs may reduce cyber-victimization more than traditional school-based bullying prevention programs alone. Aggr. Behav. 42:166–180, 2016. © 2015 Wiley Periodicals, Inc.