BackgroundWith advances in medical technologies, more children with chronic diseases are now living on into adulthood. The development of proficient self-management skills is essential for adolescents and young adults to transition from pediatric to adult health care services. An innovative way to improve the current care model and foster self-management skills could be through eHealth or mHealth (mobile health) interventions, in particular, when considering the rising ownership of digital technology by adolescents and young adults. ObjectiveThis systematic review aimed to evaluate the features, acceptability, usability, engagement, and intervention efficacy of eHealth and mHealth interventions that support self-management and health care transition in adolescents and young adults with chronic disease. MethodsThis review followed the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) reporting guidelines (registration number CRD42023378355). We systematically searched the MEDLINE complete, Embase, Cochrane Library, CINAHL complete, and ProQuest Health & Medical complete. We considered only articles published in or since 2019, as we aimed to extend the data collected by 2 previous systematic reviews. ResultsA total of 16,752 studies were screened. After removing duplicates, 14,507 studies were excluded based on the title and abstract. Ultimately, 22 studies were included. The interventions ranged from simple text messages to complex interventions involving web-based games and engagement of health care providers, which were summarized into 6 themes: medication monitoring and reminders, symptom tracking and monitoring, management goal setting, knowledge education and self-management skills training, incentives and reinforcement, and communication. Most adolescents and young adults felt the eHealth and mHealth interventions were feasible, as they were convenient, easy to use, and accessible in the context of helping manage their health. However, user engagement was variable and presented a gradual decline in youth engagement with these apps over time. Barriers that prevent user engagement are diverse, such as time-consuming video uploads, noncontinuous access to a phone, reading literacy levels, language, and false impressions. Moreover, adolescents and young adults had different preferred styles of message delivery and functions, especially the engaging elements, disease-specific information, and opportunities to communicate with peers, health care providers, and app teams. ConclusionsThere remains limited data about the effectiveness of eHealth and mHealth interventions facilitating the self-management and health care transition of adolescents and young adults with chronic diseases. Based on the available evidence, they were receptive to and interested in receiving information and managing their health using mobile apps or websites. Considering adolescents and young adults had different preferred styles of message delivery and features, to improve user engagement and provide focused interventions, it would be better to involve them early in the design process to identify their specific needs, as well as collaborate with health care providers and app teams to obtain suggestions.