The article examines the global warming of groundwater due to climate change, focusing on diffusive heat transport. The authors simulate current and projected groundwater temperatures at the global scale, projecting a conservative average warming of 2.1 °C at the water table depth (excluding permafrost regions) between 2000 and 2100 under a medium emissions pathway. Regional patterns vary significantly due to spatial variability in climate change and water table depth, with mountain regions experiencing the lowest warming rates. Groundwater temperature increases influence stream thermal regimes, groundwater-dependent ecosystems, biogeochemical processes, groundwater quality, and geothermal potential. By 2100, between 77 million and 188 million people are projected to live in areas where groundwater exceeds the highest threshold for drinking water temperatures set by any country. The study highlights the importance of groundwater as the largest store of unfrozen freshwater and its critical role in sustaining terrestrial and aquatic ecosystems, particularly in the face of climate change.The article examines the global warming of groundwater due to climate change, focusing on diffusive heat transport. The authors simulate current and projected groundwater temperatures at the global scale, projecting a conservative average warming of 2.1 °C at the water table depth (excluding permafrost regions) between 2000 and 2100 under a medium emissions pathway. Regional patterns vary significantly due to spatial variability in climate change and water table depth, with mountain regions experiencing the lowest warming rates. Groundwater temperature increases influence stream thermal regimes, groundwater-dependent ecosystems, biogeochemical processes, groundwater quality, and geothermal potential. By 2100, between 77 million and 188 million people are projected to live in areas where groundwater exceeds the highest threshold for drinking water temperatures set by any country. The study highlights the importance of groundwater as the largest store of unfrozen freshwater and its critical role in sustaining terrestrial and aquatic ecosystems, particularly in the face of climate change.