Gambling possess both entertaining and potentially harmful aspects. It remains a challenge for public health authorities and gambling operators to recommend and implement effective evidence-based interventions to reduce gambling-related harm and prevent problematic gambling behavior. This systematic review examined studies on randomized controlled trials in this field, aiming to describe and evaluate the effect of preventive and harm-reducing interventions for online gambling and electronic gaming machines. A systematic literature search was performed in PsycINFO, PubMed, Embase, and SCOPUS. Study selection, risk of bias assessment, and data extraction were conducted by two reviewers independently, and a synthesis was performed. A total of 18 studies were included, of which the majority were limited by quality issues. The applied interventions focused on restraining gambling, providing feedback, delivering therapeutic and informative content, and priming analytical thinking. The studies varied greatly in terms of specific intervention content, study setting, participants, and outcome measures, and no consistent patterns could be detected between study characteristics and related effects. Although research in the field is growing, there is still a need for further methodologically rigorous investigation and consensus on outcome measures sensitive to effects of preventive and harm-reducing interventions.