This systematic review primarily aims to provide a summary of the game mechanics implemented in eHealth tools supporting young people’s self-management of their chronic diseases. This review secondarily investigates the rationale for implementing game mechanics and the effects of these tools. A systematic search was conducted in Embase, Medline, PsycINFO, and Web of Science, from inception until August 30, 2022. Studies were eligible if focus was on the utilization of gamification in eHealth self-management interventions for young people (age = 10-25 years) with chronic diseases. Primary quantitative, qualitative, and mixed-method studies written in English were included. We identified 34 eHealth tools, of which 20 (59%) were gamified tools and 14 (41%) were serious games. We found that 55 unique game mechanics were implemented. The most commonly used were rewards (50%), score (44%), creative control (41%), and social interaction (32%). In comparison with gamified tools, the number and diversity of game mechanics applied were higher in serious games. For most tools (85%), a general rationale was provided for utilizing gamification, which often was to promote engaging experiences. A rationale for using specific game mechanics was less commonly provided (only for 45% of the game mechanics). The limited availability of experimental research precludes to test the effectiveness of using gamification in eHealth to support self-management in young people with chronic diseases. In this study, we highlight the importance of reporting the rationale for utilizing specific game mechanics in eHealth tools to ensure a proper alignment with evidence-based practice and the need of conducting experimental research. PROSPERO: CRD42021293037.@en