This study examined the effectiveness of an intervention aimed at improving parent-school cooperation in counteracting bullying. Using a randomized controlled trial, data of teachers, parents of non-victimized children, and children themselves were collected at 13 intervention and 14 control schools (grades 3-6, N at post-assessment: teachers = 83, parents = 153, children = 2,510) at two time points (time lag about 6 months). Results showed positive effects of the intervention for some aspects of the primary outcomes: parents' and teachers' attitudes and efforts, whereas no effects were found of teachers' or parents' competences in counteracting bullying. No intervention effects were found for secondary outcomes: children's self-reported bullying, victimization, well-being, and self-esteem. The findings indicate that, due to the intervention, teachers and parents were more aligned and able to cooperate, even within the short time of the intervention: one school year. This is the first essential step to systematically addressing parents' role in tackling bullying; future research is needed to examine the long-term effects of parent and school interventions in enhancing the effectiveness of anti-bullying programs.