Limited evidence exists on the association between high temperature and mental health in younger age group. We conducted a systematic search on PubMed, Web of Science, CINAHL Plus and PsycInfo and reviewed studies that explore the associations of heat exposure with mental health and suicide among children and adolescents (≤24 years). Twenty-eight observational studies published over 2007-2025 were included for systematic review. Exposure to high temperature was associated with 13% (95% confidence intervals: 1.08 to 1.19; n = 720,512) higher risk of hospital visits or hospitalizations for mental health disorders, 14% (1.01 to 1.28; n = 529,654) higher risk for schizophrenia, 18% (1.03 to 1.34; n = 146,046) higher risk for depression, and 12% (1.06 to 1.18; n = 1,188,501) higher risk for composite mental health illnesses, relative to low temperature. Each 1 °C increment in temperature was associated with 1.0% (1.00 to 1.02; n = 30,749) higher risk of suicide. Findings suggest the need for early life interventions and evidence-based adaptation policies for preventing against climate-induced mental illnesses and suicide. Optimizing policies will be important for high- and upper-middle-income countries, while more scientific evidence from lower-income economies are warranted.