Bullying is a major health problem. The KiVa antibullying program has been evaluated in Finland and other European countries, showing preventive effects on self-reported bullying victimization and perpetration. No evaluations of this program have been conducted in Latin America. A cluster randomized controlled trial was conducted at socially vulnerable schools in Santiago, Chile, to assess the effectiveness of the KiVa antibullying program in grades 5 and 6 (aged 10-12 years). Schools were randomly assigned (1:1:1) to three groups: the full KiVa group (including the online game), the partial KiVa group (did not include the online game), and the control group in which the regular school curriculum was implemented. The primary outcome was self-reported bullying victimization, assessed before the intervention (baseline) at the end of the academic year (November 2016) and post-intervention, 12 months after the baseline assessment (November 2017). This trial is registered with ClinicalTrials.gov, number NCT02898324. A total of 39 schools (13 in each group) were included; no schools withdrew. The baseline survey included 5923 participants, and the endpoint survey included 3968 participants. Participants in the partial KiVa group had lower bullying victimization at the endpoint survey than those in the control group (OBVQ-R adjusted mean difference - 0.14; 95% CI, - 0.26 to - 0.01; effect size - 0.13, 95% CI - 0.24 to - 0.01, p = 0.035). There was no effect of the full KiVa group for bullying victimization compared with the control and partial KiVa groups. Compared to the control group, participants in the partial KiVa group had lower witnessing bullying at school (adjusted mean difference = - 0.25; 95% CI - 0.45 to - 0.05; effect size - 0.18, 95% CI, - 0.32 to - 0.04, p = 0.013). No effects were found for other secondary outcomes, including bullying perpetration in any comparisons between arms. The implementation of the KiVa antibullying program had mixed results in Chile. There was only a small effect on bullying victimization and witnessing when KiVa was delivered without the online game.