Exposure to child maltreatment (CM) is considered to predispose children to devastating consequences in terms of mental health. Thus, it is a public health priority to provide these children with early preventive interventions that are accessible on a large scale, adapted to their needs, and effective in supporting their mental health. Here we report a randomized control trial to test the efficacy of the REThink online therapeutic game, as compared with a Care as Usual (CAU) control group in the prevention of mental illness in maltreated children. Out of 439 children aged 8–12 that were recruited, 294 children with self-reported maltreatment histories were included in the current study, and were allocated, 146 participants in the REThink group and 148 participants in the CAU group. All children completed pre- and post-intervention assessments measuring mental health, emotion regulation, and irrational cognitions. We also tested potential moderators for these effects, such as the severity of CM and the security of parent attachment. Our results show that children receiving the REThink game intervention outperform the CAU group at post-test, showing a significantly lower level of emotional problems, mental health difficulties, use of maladaptive emotion-regulation strategies such as catastrophizing, rumination, and self-blame, and irrational cognitions. Moreover, children with higher CM severity benefit the most from the REThink game, while children with lower parent attachment security benefit the least. Future research is needed, to investigate the long-term efficacy of the REThink game in promoting the mental health of children exposed to CM.