Coastal aquifers act as a major host of seawater-groundwater interaction and play an important role in modulating the marine elemental budget. Calcium stable isotopes (δ^44/40Ca) have been used to identify mass dependent isotope fractionation processes such as carbonate dissolution and precipitation in a range of geological reservoirs that has major implications in constraining global geochemical cycles. However, there is limited Ca isotope data from coastal aquifers globally. Here we report δ^44/40Ca values of groundwater collected in 2017-18 from multiple locations and depths from the Bakkhali delta front, Sundarbans, India. The sampling depth varied between 14 m below ground level (m bgl) and 333 m bgl and the salinity of the groundwater samples range from 1-25 ppt. The salinity of the water samples decreases with increasing depth indicating greater seawater incursion from the Bay of Bengal at shallower depths. Variable amounts of mixing of freshwater and seawater is also supported by Sr and Ca concentrations which vary between 1.6-62.8 μmol/l and 0.29-8.92 mmol/l, respectively, and show progressively lower concentrations with depth. The δ^44/40Ca values of dissolved phase in groundwater samples (relative to NIST SRM 915a) were measured using a ⁴³Ca-⁴⁸Ca Double Spike TIMS technique at Centre for Earth Sciences, Indian Institute Science, Bangalore. The δ^44/40Ca values of the groundwater samples show significant variability between 1.52-2.28‰ (2SD ~0.1‰) with several samples showing δ^44/40Ca values higher than modern seawater (~1.88‰). Low δ^44/40Ca values, mostly in deeper groundwater samples, is consistent with higher proportions of freshwater input. Samples showing high δ^44/40Ca values are mostly from shallower depths and likely reflect carbonate precipitation which is consistent with high Sr/Ca (~8.13-12.26) in samples from 30-42 m depth. The high δ^44/40Ca in groundwater samples from the Ganges-Brahmaputra delta could explain the high δ^44/40Ca values reported in water samples from the Bay of Bengal¹. [1] Chakrabarti et al., Goldschmidt Boston, 2018 Abstract.