Droughts in South Asia pose substantial challenges to water resources, agricultural and socio-economic growth of more than two billion people. However, the most detrimental soil moisture droughts, their impacts and occurrence in the observed and projected climate in South Asia remain unidentified. We conducted soil moisture simulations using the variable infiltration capacity model and meteorological forcing based on the four global observational products (Climate Research Unit, WATCH-Forcing-Data-ERA-Interim, University of Delaware and Princeton) to identify the worst soil moisture drought during 1951-2016. The worst soil moisture drought during the observed record occurred in 2002 and affected more than 65% of South Asia and resulted in a decline in rice yield (∼400 kg ha-1). A large uncertainty based on the four observational products was found in the number of drought spells and their characteristics. We estimated the frequency of droughts that exceeded the overall severity score of the worst drought during 1951-2016 under the future climate using 40 simulations from Community Earth System Model-Large Ensemble. The frequency of the worst droughts is projected to increase by about 1.5 times in the future (2035-2100) in comparison to the historic (1951-2016) climate. The worst droughts in the future are likely to be more intense and widespread in South Asia. A considerable influence of industrial aerosols was found on the occurrence of the worst droughts in South Asia. Our findings have implications for food and water security in one of the most densely populated regions in the world.