The Congo Basin is of global significance for biodiversity and the water and carbon cycles. However, its freshwater availability remains highly unknown. Here, we leverage a new method to characterize the relationship between drainable water storage and river discharge across the entire basin. We estimate that the Congo Basin currently holds 481±24 km³ of Total Drainable Water Storage, unevenly distributed throughout the region, with 68% being stored in the southernmost sub-basins, Kasaï (227±17 km³) and Lualaba (146±7 km³), while the northern sub-basins contribute only 173±8 km³. We further estimate the time constant for draining its entire water storage to be 4.3±0.4 month, but, regionally, large lakes act as resistors resulting in greater time constants of up to 105±3 month. Our estimate provides a robust basis to address the challenges of water demand for 120 million inhabitants, a population expected to double in a few decades.