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Heat exposure is an increasingly important climate-related determinant of mental health, yet the biological mechanisms linking heat to psychiatric outcomes remain insufficiently integrated. This narrative review synthesizes evidence from climate medicine, thermoregulation, neuroscience, psychoneuroimmunology, sleep research, psychopharmacology, epidemiology, and psychiatry to examine how acute and chronic heat exposure may influence mental health. Heat acts as a multi-system biological stressor by challenging thermoregulation, increasing autonomic and cardiovascular strain, disrupting hydration, impairing sleep and circadian rhythms, activating stress-response systems, and contributing to neuroinflammation, oxidative stress, mitochondrial dysfunction, blood-brain barrier disruption, and neurotransmitter alterations. These pathways may interact with psychotropic medication exposure and impaired behavioral thermoregulation, particularly among people with severe mental illness, older adults, individuals receiving lithium, people with cognitive impairment, substance use disorders, outdoor workers, and those living in poorly cooled environments. Heat-related psychiatric vulnerability is therefore transdiagnostic but heterogeneous, with relevance to distress, depression, anxiety, mood instability, psychosis, substance-related crises, cognitive dysfunction, delirium, and suicide-related outcomes. The review also distinguishes pathways supported by stronger human evidence from mechanisms supported mainly by experimental, preclinical, severe-spectrum, or indirect evidence. It proposes an integrative biological model and an evidence-informed translational agenda focused on exposure assessment, biomarkers, sleep and circadian monitoring, medication safety, risk prediction, physiological adaptation, heat-mitigation strategies, heat-health action plans, and implementation-oriented evaluation.