We examine the effects of ambient temperatures on mental health using a nationally representative longitudinal survey of Chinese individuals. We find that temperatures over 30?C significantly increase the likelihood of depression. High temperatures have larger detrimental effects on the mental health of the middle-aged and elderly, females, the less-educated, and agricultural workers. We discuss two likely mechanisms for the mental health impact of high temperatures: raising the incidence of physical illness and reducing sleeping time. We find suggestive evidence of air conditioners moderating the adverse impacts of high temperatures and of adaptation to high temperatures in the long term. We reveal that without any government interventions or private adaptation, mental health will deteriorate by 3.1% in the medium term and 5.3% in the long term based on the Hadley GEM2-ES climate-change projection.