Background and aims: Despite its illegality among adolescents, online gambling is a common practice, which puts their mental health and well-being at serious risk. This systematic review summarises international scientific literature from the last 20 years on problematic online gambling among adolescents (11-21 years old) to determine its prevalence and to analyse related measurement issues. Methods: The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed and a protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO, IC: CRD42020162932). Five academic databases were consulted, which resulted in an initial sample of 658 papers. Results: Sixteen studies met the inclusion criteria for this review. All studies were cross-sectional and targeted students from elementary school, secondary school or university. Most followed a convenience sampling procedure. The primary measurement instruments used were the DSM-IV-MR-J and SOGS-RA. Between 0.77% and 57.5% of adolescents present some degree of problematic online gambling (problem, pathological or disordered) depending on the instruments used, the study samples and the timeframe analysed. Between 0.89% and 1% of adolescents exhibited an online gambling disorder. Discussion and conclusion: There is a great heterogeneity in the methodology of the reviewed studies (samples, measurement instruments, cut-off points and criteria applied). The limited number of studies and the limited generalizability of their results suggest the need for further research and for development of specific instruments to assess different levels of problematic online gambling in representative samples of adolescents based on clinical 'gold standard' criteria and more accurate cut-off points. Kulcsszavak: online gambling, adolescents, disorder, pathological, problem, systematic review