Bystanders of cyberbullying play an important role in the resolution of such situations and therefore, it is beneficial to promote self-regulation strategies that enable them to engage in prosocial behavior in these contexts. We propose that serious game-based psychosocial interventions with profile-based social agents can encourage prosocial bystander behavior in cyberbullying. A pilot quasi-experimental study with repeated and pre/post measurements was performed. We randomly assigned 194 7th and 8th graders to three conditions, namely experimental condition (n = 103, M age = 13.91, SD = 1.02, 53.3% male); alternative condition (n = 37, M age = 14, SD = 0.86, 54.1% female) and control condition (n = 54, M age = 13.92, SD = 0.85, 50.9% female). An analysis of covariance showed that players revealed higher levels of prosocial assertive behavior when compared to other participants. Through multilevel modelling of longitudinal log-file data, we found that those who did not experience the game tended to interpret the cyberbullying situations more as non-serious, avoid assuming responsibility for intervening, and engage in aggressive behavior toward the victim. Players tended to support more and were less aggressive with victims from their in-group than those from the out-group. Insights for the development of games to promote prosocial behavior in bystanders of cyberbullying are presented. P. C. Ferreira et al. / Psychosocial Intervention (2022) xx(x) xx-xx