This study evaluates the effectiveness of the KiVa antibullying program in the Netherlands through a randomized controlled trial of students in grades 3-4 (Dutch grades: 5-6). The sample involved 98 schools who volunteered to participate in the research, with 245 classes and 4,383 students at the baseline (49% girls; M age = 8.7 years), who participated in five measurement waves, collected in three consecutive school years. After the baseline, two thirds of the schools were assigned to the intervention condition (KiVa or KiVa+, the latter included an additional intervention component of network feedback to teachers) and one third to the control condition (waiting list, care as usual) with a stratified randomization procedure. The effects of the intervention on self-reported victimization and bullying were tested using cross-classified ordered multinomial models and binomial logistic regression models. These longitudinal models showed that self-reported victimization and bullying reduced more strongly in KiVa-schools compared to control schools, with stronger effects after two school years than after one school year of implementation. The results showed that for students in control schools, the odds of being a victim were 1.29-1.63 higher, and the odds of being a bully were 1.19-1.66 higher than for KiVa students. No significant differences between KiVa and KiVa+ emerged. Overall, the findings provide evidence of the effectiveness of the KiVa program in the Netherlands.