Background: Although gaming is a common arena where children socialize, an increasing number of children are exhibiting signs of problem gaming or internet gaming disorder. An important factor to the development of problem gaming is parent-child relationships. A cognitive behavioral therapy–based form of treatment, labeled relapse prevention, has been developed as a treatment for child and adolescent problem gaming or internet gaming disorder. However, no study has evaluated the effect of this treatment among Swedish children and youth nor the role of the parent-child relationships in this treatment. Objective: This study aims (1) to evaluate a relapse prevention treatment for patients showing signs of problem gaming or internet gaming disorder recruited from child and youth psychiatric clinics and (2) to test whether the quality of parent-child relationships plays a role in the effect of relapse prevention treatment and vice versa—whether the relapse prevention treatment has a spillover effect on the quality of parent-child relationships. Moreover, we explore the carer’s attitudes about parent-child relationships and child gaming, as well as experiences of the treatment among the children, their carers, and the clinicians who carried out the treatment. Methods: This study is a 2-arm, parallel-group, early-stage randomized controlled trial with embedded qualitative components. Children aged 12-18 years who meet the criteria for problem gaming or internet gaming disorder will be randomized in a 1:1 ratio to either intervention (relapse prevention treatment) or control (treatment as usual), with a total of 160 (80 + 80) participants. The primary outcomes are measures of gaming and gambling behavior before and after intervention, and the secondary outcomes include child ratings of parent-child communication and family functioning. The study is supplemented with a qualitative component with semistructured interviews to capture participants’ and clinicians’ experiences of the relapse prevention, as well as attitudes about parent-child relationships and parenting needs in carers whose children completed the treatment. Results: The trial started in January 2022 and is expected to end in December 2023. The first results are expected in March 2023. Conclusions: This study will be the first randomized controlled trial evaluating relapse prevention as a treatment for child and adolescent problem gaming and internet gaming disorder in Sweden. Since problem behaviors in children interact with the family context, investigating parent-child relationships adjacent to the treatment of child problem gaming and internet gaming disorder is an important strength of the study. Further, different parties, ie, children, carers, and clinicians, will be directly or indirectly involved in the evaluation of the treatment, providing more knowledge of the treatment and its effect. Limitations include comorbidity in children with problem gaming and internet gaming disorder and challenges with the recruitment of participants.