Cyberbullying is a social phenomenon which can bring severe harm to victims. Bystanders can showpositive bystander behavior (e.g. defending) and decrease cyberbullying and its harm, or negativebehavior (e.g. passive bystanding, joining) and sustain cyberbullying and its negative effects. Few interventionshave currently targeted bystanders and evaluated results on their behavior or its determinants.The intervention consisted of a serious game specifically targeting cyberbullying bystanderbehavior. A cluster-randomized controlled trial was conducted among 8th graders (n ¼ 216) in twoschools. Measurements were taken at baseline, immediately after the intervention and at 4-week followup.The serious game intervention resulted in significant improvements in self-efficacy, prosocial skills,and the intention to act as a positive bystander. These are mainly predictors of positive bystanderbehavior. No significant effects were found for predictors of negative bystander behavior. The interventionalso increased witnessing of cyberbullying incidents, potentially a measure of awareness ofcyberbullying taking place, and quality of life. No effects were found on behavior itself, bullying orcyberbullying prevalence. This brief serious game intervention affected determinants of bystanderbehavior and quality of life among adolescents. Further efforts are needed to address (negative)bystander behavior and cyberbullying involvement.