Groundwater is an essential freshwater resource for sustaining groundwater-dependent ecosystems, maintaining streamflow during low-flow periods, and meeting anthropogenic water demands. With increasing pressures from climate change and water use expected to intensify groundwater stress throughout the 21^st century, it is valuable to evaluate the temporal evolution of projected groundwater sustainability. Here, we project global groundwater sustainability from 2026&ndash;2100 using a coupled VIC-WUR-MODFLOW modelling framework forced with bias-adjusted climate projections and future water use scenarios under SSP1-2.6 and SSP3-7.0. Using an environmental flow-based approach, we calculated monthly groundwater sustainability thresholds based on <em>natural</em> conditions. We compared simulated output of groundwater head under <em>human-impacted</em> conditions to these thresholds to quantify the frequency, severity, timing, and duration of unsustainable groundwater use at 5-arcmin spatial resolution. Our results project groundwater head to drop below environmentally sustainable thresholds at least once from 2026&ndash;2100 in ~90% of evaluated gridcells, demonstrating that unsustainable groundwater use, leading to harmful ecosystem impacts, is likely to be widespread throughout the 21^st century. Despite its spatial prolificness, groundwater unsustainability is primarily characterized by recurring violations rather than permanent threshold exceedances. Global medians of frequency and longest continuous duration of unsustainable conditions are 30% and 40 months respectively. The most severe violations of sustainable conditions are concentrated within a limited number of hotspot regions, including the Indo-Gangetic Plain, North China Plain, Central Valley, and High Plains. Distinct seasonal patterns also emerge across many aquifers, and unsustainable conditions generally increase during the late-century (2076&ndash;2100). Between SSP1-2.6 and SSP3-7.0, the main difference in our findings is in projections of unsustainable severity, indicating that future climate and socioeconomic pathways may primarily influence the degree to which groundwater systems drop below environmentally sustainable conditions rather than whether these thresholds are exceeded. By explicitly resolving seasonal, inter-annual, and decadal dynamics of global groundwater sustainability, this study provides a global-scale framework for identifying future hotspots of groundwater overexploitation which could guide groundwater management strategies aimed at supporting environmental, hydrogeological, and anthropogenic water demands.<span></span>