Inter-basin groundwater flow (IGF) transfers water across watershed boundaries and interacts with surface hydrological processes. However, the spatiotemporal characteristics of IGF in major river basins worldwide and its key role in the hydrological cycle remain unclear. Here, we reconstructed IGF time series for 36 major global basins. The results show that the absolute magnitude of the multi-year mean IGF exceeded 20 % of precipitation in nearly half of the basins. Neighboring basins show similar seasonal patterns of IGF, reflecting the response of lateral groundwater flow to climatic seasonality. More importantly, in almost all basins, the rapid response of IGF to precipitation is consistent with pressure propagation through groundwater systems. In half of the basins, IGF could serve as an external water source supporting evapotranspiration. These findings demonstrate that IGF is a critical link in the hydrological cycle. Neglecting IGF may introduce structural biases into the representation of hydrological processes and thus reduce the reliability of climate-change projections based on Earth system models.