In the scenario of global warming and climate change, human thermal discomfort is about to rise. A rise in human thermal discomfort will undermine human health and well-being. It will also undermine labour productivity (as workers have to reduce work intensity and take longer breaks from work to prevent heat stress-related illness and injuries) and boost energy demand (as people will have to use more cooling instruments such as ACs, coolers, fan, etc., to get relief from thermal discomfort). Hence an assessment of spatio-temporal variability of thermal discomfort is necessary to develop a national strategy for the sustainable development of the country under changing climate scenarios. In this study, we have tried to analyze spatio-temporal variations of summertime thermal discomfort in India with the help of the Discomfort Index (DI). To calculate the DI, we have used high resolution (0.25 degrees x0.25 degrees) ERA-5 hourly 2-m air temperature and 2-m dewpoint temperature data. It is seen that March is the month of minimum discomfort and June is the month of maximum discomfort. In June, maximum discomfort occurs in the western region. The east coastal region and western region of India, particularly Rajasthan, experience maximum discomfort in terms of severity and prolonged discomfort hours. We have also calculated trends in DI, RH and temperature over the Indian region for March to June and observed a generally increasing trend with some spatial variations across India. It is also observed that the DI trend is more prominent in the western region in March and April, the southern region in May and the eastern region in June. We have also calculated the diurnal variations of thermal discomfort and the number of days with DI greater than 27 degrees C and 29 degrees C for different regions. It is observed that in most of the regions, DI reaches its peak around 09-10Z. Except for the north region, most of the regions show increasing trends in the number of discomfort days in April, May and June.