Heat uptake is a key variable for understanding the Earth system response to greenhouse gasforcing. Despite the importance of this heat budget, heat uptake by inland waters has so far not beenquantified. Here we use a unique combination of global-scale lake models, global hydrological models andEarth system models to quantify global heat uptake by natural lakes, reservoirs, and rivers. The total netheat uptake by inland waters amounts to 2.6±3.2×1020J over the period 1900–2020, corresponding to3.6% of the energy stored on land. The overall uptake is dominated by natural lakes (111.7%), followed byreservoir warming (2.3%). Rivers contribute negatively (-14%) due to a decreasing water volume. Thethermal energy of water stored in artificial reservoirs exceeds inland water heat uptake by a factor∼10.4.This first quantification underlines that the heat uptake by inland waters is relatively small, butnon-negligible.