The urban heat island (UHI) effect leads to increased heat exposure among the population in urban areas. It results from a combination of specific meteorological conditions and certain urban characteristics. This study examined the influence of certain urban characteristics conducive to the formation of UHIs on the relationship between temperature and mortality in municipalities across the Île-de-France region. The analysis focused on total mortality during the summers (June–September) from 2000 to 2015. Daily death data for each municipality were modeled against the daily average temperature, controlling for the social deprivation index, the proportion of elderly residents, and four urban indicators: non-vegetated artificial surface area, non-tree canopy cover, imperviousness rate, and the proportion of the population living in an area with moderate or high potential for nocturnal HIC. In Paris and the inner suburbs, only indicators explicitly related to vegetation influence the risk of death associated with a temperature at the 95th percentile of the temperature distribution. At the 99th percentile, all tested indicators influence the risk of death, with lower relative risks (RR) of death relative to the median temperature in municipalities with more vegetation, more trees, or less impervious surface. For example, the RR is 2.17 [1.98; 2.38] when the municipality has 60% non-forested land cover, and 2.53 [2.44; 2.62] when the municipality has 93% non-forested land cover. In the outer suburbs, the indicator of non-forested area does not influence the risk of heat-related deaths. However, lower RR values are observed in municipalities with more vegetation and less artificial development. The results highlight that urban characteristics contributing to the Urban Heat Index (UHI) are associated with a higher risk of heat-related deaths. Interventions targeting these characteristics, particularly regarding vegetation and soil sealing, could therefore help reduce the health impacts of extreme heat. They must be combined with other measures to prevent the effects of heat.