Extreme temperatures are known to cause adverse health outcomes. Yet knowledge on the magnitude of this effect in developing countries is limited due to data availability and reliability issues. Collecting data for 2,872 counties in China, we estimate the effects of daily temperatures on the monthly mortality rate. The results indicate that an additional day for which the maximum temperature exceeds 38°C on average increases the monthly mortality rate by about 1.7% relative to if that day’s maximum temperature had been in the range 16–21°C. This is after deducting deaths brought forward from the subsequent month. Higher gross domestic product per capita at the county level is associated with lower mortality effects of hot and cold days. Improved dwelling conditions are found to be associated with a lower mortality effect of hot days, and improved local healthcare infrastructure to be associated with a lower mortality effect of cold days. In the absence of adaptation efforts, the estimates suggest net upward pressure on mortality rates over coming decades in some counties, especially under more extreme climate change scenarios.