Hydropower and thermoelectric power together contribute 98% of the world’s electricity generation at present. These power-generating technologies both strongly depend on water availability, and water temperature for cooling also plays a critical role for thermoelectric power generation. Climate change and resulting changes in water resources will therefore affect power generation while energy demands continue to increase with economic development and a growing world population. Here we present a global assessment of the vulnerability of the world’s current hydropower and thermoelectric power-generation system to changing climate and water resources, and test adaptation options for sustainable water-energy security during the twenty-first century. Using a coupled hydrological-electricity modelling framework with data on 24,515 hydropower and 1,427 thermoelectric power plants, we show reductions in usable capacity for 61-74% of the hydropower plants and 81-86% of the thermoelectric power plants worldwide for 2040-2069. However, adaptation options such as increased plant efficiencies, replacement of cooling system types and fuel switches are effective alternatives to reduce the assessed vulnerability to changing climate and freshwater resources. Transitions in the electricity sector with a stronger focus on adaptation, in addition to mitigation, are thus highly recommended to sustain water-energy security in the coming decades.