The heatwave in 2022 in South Asia disrupted the lives of millions of people and posed challenges to human health, energy, water, and food security. However, mega heatwaves' causes, impacts, and occurrence (like in 2022) remain largely unrecognized. Here, we analyzed the 2022 heatwave, its mechanisms, and future likelihood using observational datasets and climate model simulations. In the last few years, the frequency and duration of heatwaves have significantly increased in South Asia. South Asia faced five continuous heatwave spells that lasted about 35 days during late February and April 2022, affecting a large part of the region. The year 2022 heatwave was unprecedented that caused a deficit in soil moisture and crop yield. Moreover, our results show that the excessive radiative heating of arid and semi-arid regions resulted in a high geopotential height and low pressure in South Asia during the 2022 mega heatwave. The climate model simulations show that such mega heatwaves are projected to become more frequent under the warmer world, and their time of emergence could be as early as the 2030s under the highest emission scenario. Earlier occurrences of mega heatwaves in the future will pose considerable adaptation challenges for food and water security in the region.