In India, the likelihood of intense heat waves reported in 2022 is projected to increase 30-fold with continued global warming. India’s National Programme on Climate Change and Human Health (NPCCHH) aims to strengthen health sector preparedness to reduce morbidity and mortality from impacts of climate change, including extreme heat. Mass gatherings during summer heighten the risk of severe heat-related illnesses (HRI). A coastal district in Maharashtra state reported deaths during a mass-gathering event in April 2023, prompting an investigation and situational analysis. We conducted a cross-sectional study in the district. We defined confirmed heatstroke deaths as suspected HRI deaths among attendees of the April 16th, 2023, mass gathering, confirmed through post-mortem examination. Medical records were reviewed, and epidemiological data were collected using NPCCHH’s suspected HRI death investigation form from relatives and first responders. Data were analysed in proportions. The district reported 12 confirmed heatstroke deaths at a mass gathering of one million attendees on April 16th, 2023. Of these, 11 were found dead within two hours of crowd dispersal. There was no active heatwave warning. Disaster preparedness, although extensive, focused on managing generic scenarios. No information on identifying and preventing HRI was publicised. The heatstroke deaths highlighted policy and implementation gaps in organizing mass gatherings during summer. Recommendations include developing mass gathering-specific preparedness guidelines considering ambient heat exposure.