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Cities have been assigned as one of the most vulnerable areas with respect to heat-related risk due to global warming and rapid urban growth. The present study explores the long-term trends in thermal risk at a large urban area of the eastern Mediterranean (Athens) over a long period (1960-2017), based on hourly observations. In addition to the frequency and severity of heat stress conditions, the study further explores changes in the seasonality of heat stress. Four human thermal indices with different rationales were employed, namely the Universal Thermal Climate Index (UTCI), the Physiologically Equivalent Temperature (PET), the Heat Index (HI), and the Humidex (HD). All indices indicate a prominent increase in heat-related risk over the years. The exposure time per year under the conditions of "hot-extreme caution" (HI), "great discomfort-avoid exertion" (HD), "very strong heat stress" (UTCI), and "extreme heat stress" (PET) exhibits a statistically significant increasing trend at a rate of 0.9%/decade, 0.4%/decade, 0.3%/decade, and 0.4%/decade during 1960-2017, respectively. Even during the nighttime hours, three out of the four indices indicate that the population is exposed to significantly higher heat stress levels in the recent decades compared to the past ones. A progressive expansion of the "heat stress season" over the years was revealed, resulting to an elongation of the "hot-extreme caution" season (HI), the "great discomfort-avoid exertion" season (HD), and the "very strong heat stress" season (UTCI) by 5.6 days/decade, 11.3 days/decade, and 4.3 days/decade, respectively.