The isotopic composition of water has been widely used to investigate groundwater dynamics and recharge processes. In the present study, we have analysed the isotopic composition of groundwater, and of Sutlej River, Yamuna River, Ghaggar River and canal water from the Sutlej-Yamuna plain in NW India. We document spatial and depth-related variations in δ¹⁸O and deuterium excess (d-excess) values of groundwater in three zones based on topography and slope (zones I–III) from NE to SW. In Zone I, precipitation is the main recharge source for groundwater, as indicated by the isotopic values. We infer mixed recharge from precipitation and irrigation return flow in Zone II. Zone III records depleted δ¹⁸O and higher d-excess values in most of the groundwater samples, suggesting active recharge from canals. Further, we have calculated the contribution of canal water in groundwater using the hydrograph separation approach and have also quantified the uncertainty in its estimation. We note significant spatial and depth-related variability in the canal water contribution to groundwater recharge. The canal contribution is as high as 83 ± 10% at 18 m below ground level (bgl) in Zone III, and as low as 5 ± 3% at 43 m bgl in Zone II. The present study provides the baseline data on recharge processes in NW India, which is critical for developing sustainable management strategies for groundwater resources in this region.