Abstract Water competition between the food and energy sector is a critical component of the food-energy-water nexus. However, few studies have systematically characterized the geospatial and, especially, the sub-annual variations in such competition and the associated environmental impacts and targeted mitigation opportunities. This study characterizes competing water uses for crop-specific irrigated agriculture and fuel-specific power generation across global major river basins to reveal their resulting impacts on local water scarcity for global population under both current and a warming climate. Under annual (and most seasonal) accounting, almost all basins currently suffering from extremely high water scarcity are dominated by agricultural water consumption (e.g. accommodating 26%–49% of basin-total population across seasons), which are often simultaneously exposed to potentially decreasing seasonal water availability under a 4 °C warming scenario. Only 13%–20% of population are located in basins dominated by seasonal power sector water uses, which are predominantly with low water scarcity. Agriculture sector provides the most basin-specific water mitigation opportunities across mid-latitude basins in all four seasons. Nevertheless, power sector becomes more important in affecting seasonal water scarcity and provides unique seasonal water mitigation opportunities, particularly in basins among higher northern latitudes in winter. This analysis highlights irrigated agriculture is currently and will likely remain the key in global water management for basins facing the severest water scarcity, yet increasing attention on the seasonal and spatial variations in cross-sector water use competition is needed to better identify region- and season- specific mitigation opportunities.