High temperatures are identified by the Intergovernmental Panel on Climate Change (IPCC) as one of the most concerning climate risks in Europe, given their effects on human health and ecosystems. In mainland France, heat waves are the extreme weather events associated with the highest human burden, with more than 42,000 excess deaths from all causes observed during these events between 1970 and 2022. Beyond heat waves, high temperatures—which are often perceived as posing no health risk—also present significant risks. Since heat has multiple effects on health, it is not possible to identify heat-attributable deaths in real time based, for example, on a single predefined cause of death. This report presents a method for quantifying, using historical data, each year at the departmental level, the proportion of mortality attributable to heat during the summer (more specifically during the monitoring period of the heatwave and health alert system, from June 1 to September 15), in order to supplement assessments specifically targeting the estimation of all-cause mortality during heatwave periods. Temperature-mortality relationships applicable to French metropolitan departments were developed for the June–September period from 2014 to 2022. In each department, these relationships link the average temperature measured by a Météo-France reference station and total mortality (all ages and those aged 75 and older) recorded by INSEE. They account for seasonality and long-term trends, as well as the potential influence of the COVID-19 pandemic (starting in 2020). These relationships were then used to estimate, for each year, the proportion of mortality attributable to heat between June 1 and September 15 for the years 2014–2022. Heat is defined here as a temperature above the summer median of the 2014–2022 temperature distribution. Between 2014 and 2022, across all metropolitan departments, 32,658 deaths [95% confidence interval (CI) = 29,612: 34,975] were attributable to heat between June 1 and September 15 of each year, including 23,080 [95% CI = 21,076: 24,556] deaths among people aged 75 and older. Twenty-eight percent of these deaths occurred during heat waves as defined by the heat wave management plan. The greatest impacts were observed in 2022 (6,969 [6,277:7,445] excess deaths, 29% of which occurred during heat waves), and 2019 (4,441 [4,086:4,717] excess deaths, 42% of which occurred during heat waves), with regional variation. The impact also exceeded 4,000 deaths in 2018 and 2020. The results highlight the significant impact of heat on mortality since 2014. As expected, approximately two-thirds of the impact primarily affects people aged 75 and older, but it should be noted that a significant portion (one-third) affects people under 75. The focus on heatwave alerts is justified by their contribution to the overall toll: on average, 6% of days corresponding to heatwave periods account for 28% of the heat-related impact. Organizing a specific response during heatwaves is also necessary, given their potential for massive and rapid disruption of the healthcare system, as observed in 2003. However, this reactive adaptation during extreme events must be complemented by structural and systemic adaptation to heat to reduce the risk to human health throughout the summer.