Abstract Hydrological extremes are projected to intensify under climate change, yet their implications for water scarcity remain poorly understood. We use a multi‐model ensemble to assess how changes in hydrological low and high extremes will constrain renewable water for human use across 520 major river basins worldwide. We evaluate trends in two metrics of water availability: (a) annual lowest monthly runoff, and (b) annual accessible runoff, a conceptual indicator defined as total runoff excluding extreme high flows typically released rapidly for flood control. More than 40% of river basins are projected to experience declines in both metrics during 2015–2100. Mediterranean and several scattered basins are identified as hotspots where runoff reductions compound existing water scarcity. Scenario comparison shows that low emission mitigates, though do not fully eliminate, these declines, highlighting hydrological co‐benefits of climate mitigation. The novelty lies in explicitly incorporating hydrological extremes into water availability assessment.