Ambient temperatures have an impact on human health, with unfavourably warm and cold conditions both associated with elevated mortality risk. By modulating the temperature in urban environments, urban heat islands (UHIs) can therefore both amplify the impact of heat and offer protection against cold weather. In this study, we quantify the impact of UHI on human mortality at 500m resolution for 85 European cities using air temperature simulations and age-dependent epidemiological temperature-mortality relationships for each city. On an annual basis, UHIs have weak net protective effects for most cities examined. This is due to the prevalence of cold to mild days in these cities when an increase in temperature is associated with slight reductions in mortality risk. On a daily basis, however, UHIs induce the greatest impact during heat extreme days, with a median of 39% increase in risk compared to a 7% reduction during cold extreme days. A valuation of such mortality risk reveals that the annual cost of UHI-related heat mortality is comparable to air pollution-related mortality costs as well as transit costs. Cities with Arid climates and Temperate Dry Summer climates in Southern Europe tend to experience the greatest protective UHI effects during cold extreme weather and the least adverse effect during heat extremes, while cities with Cold climates in Eastern and Northern Europe tend to benefit the least during cold extremes. Annually, the net impact of UHI is most strongly correlated with each city's vulnerability to heat and cold and the ratio of warm vs. cold days in a year.