This study was performed to elucidate the distribution, concentration trend and possible sources of total mercury (HgT) and methylmercury (MeHg) in sediment cores (<63 μm particle size; n = 75) of Sundarban mangrove wetland, northeastern part of the Bay of Bengal, India. Total mercury was determined by atomic absorption spectrometry (AAS) in a Leco AMA 254 instrument and MeHg by gas chromatography–atomic fluorescence spectrometry (GC-AFS). A wide range of variation in HgT (0.032–0.196 μg g−1 dry wt.) as well as MeHg (0.04–0.13 ng g−1 dry wt.) concentrations revealed a slight local contamination. The prevalent low HgT levels in sediments could be explained by sediment transport by the tidal Hugli (Ganges) River that would dilute the HgT values via sediment mixing processes.