The present study attempts to generate the first baseline data on gross α and β activities in groundwater and riverine water samples collected from different regions of Indian Sundarbans, a part of the world's largest mangrove ecosystem. Until the present, no information is available related to radioactivity measurement in water samples from this vast area. Gross alpha-beta activities were measured by liquid scintillation counting-triple to double coincidence ratio (LSC-TDCR) technique. The minimum detectable activities in present experimental condition were found to be 21 mBq L−1 and 55 mBq L−1 for gross α and β respectively. Gross alpha activities in all groundwater and riverine samples were found to be below the detection limit (BDL), whereas gross beta activities in groundwater and riverine samples varied from BDL to 0.46 ± 0.24 Bq L−1 and BDL to 0.90 ± 0.26 Bq L−1 respectively, which are below WHO recommended value 1 Bq L−1. Annual effective doses were below 0.1 mSv. U and Th concentrations in the water samples were determined by ultrasonic-nebulizer assisted Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and found to be BDL. For assessing 3H activity, double-distilled water samples were measured by LSC-TDCR technique, which provided BDL result. © 2020 Elsevier Inc.