Purpose of Review: This narrative review synthesized recent research related to obesity in adolescents with psychiatric disorders, with a focus on epidemiology, mechanisms, and weight management approaches. The paper reviews literature on depressive and anxiety disorders, bipolar disorder, and schizophrenia spectrum and other psychotic disorders. Recent Findings: Depression has a bidirectional relationship with obesity. Bipolar disorder and schizophrenia spectrum disorders, and their treatments, increase the risk of developing obesity. Mechanisms underlying this weight gain include lifestyle and environmental factors and psychiatric medications, though emerging evidence has also suggested the role of genetic and neuroendocrine processes. Evidence about the most effective treatments for obesity in adolescents with psychiatric disorders remains limited. Summary: Adolescents with psychiatric disorders are at high risk for obesity. Close monitoring for increases in weight and cardiometabolic risk factors with use of antipsychotic and moodstabilizing medications is recommended. Clinical trials are needed that test the efficacy of weight management strategies for this population.