Background: Although adherence to somatic treatment has been studied extensively, adherence to mental health treatment has not. In this study, the term treatment adherence is used to refer to adherence to medication regimes and other non-pharmacological recommended treatments as part of mental health and psychiatric care. Knowledge of factors connected to adolescents' adherence to such treatment is fairly fragmented. Although treatment staff members are broadly aware of the factors that influence adherence, it would be desirable to develop formalized treatment assessment and planning approaches that specifically take these factors into account. Objective: The purpose of this article is to review the available data related to factors that affect adolescents' adherence to both medication and non-pharmacological treatments in mental health and psychiatric care. Methods: A systematic literature search that involved four databases was undertaken. A thematic analysis was conducted on 17 studies to extract relevant information. Results: It appears that an adolescent's own positive beliefs toward his or her treatment are the main factor that improves his or her adherence to that treatment. Good cohesion with close supportive people appears to be positively related to adherence, whereas sociodemographic characteristics (e.g., race, gender) were not related. Conclusions: By identifying the factors related to the treatment adherence of adolescents, this review article can provide guidance to help improve the quality of care and thus further increase adolescents' satisfaction with their mental health and psychiatric treatment.