Biogeochemical cycling of CH4 was investigated at Lothian Island, one of the relatively pristine islands of Indian Sundarbans and its adjacent Saptamukhi estuary, during June 2010 to December 2012. Intertidal mangrove sediments were highly anoxic and rich in organic carbon. Mean rates of methanogenesis were 3,547 and 48.88 μmol m-3 wet sediment d-1, for intertidal (up to 25 cm depth) and sub-tidal sediments (first 5 cm depth), respectively. CH4 in pore-water was 53.4 times more supersaturated than in adjacent estuarine waters. This resulted in significant CH4 efflux from sediments to estuarine waters-via advective and diffusive transport. About 8.2% of the total CH4 produced in intertidal mangrove sediments was transported to the adjacent estuary through advective flux, which was 20 times higher than diffusive CH4 flux. Mean CH4 concentrations in estuarine surface and sub-surface waters were 69.9 and 56.1 nM, respectively, with a dissolved CH4 oxidation rate in estuarine surface waters of 20.5 nmol L-1 d-1. An estimated 0.09 Gg year-1 of CH4 is released from estuaries of Sundarbans to the regional atmosphere. The mean CH4 mixing ratio over the forest atmosphere was 2 ppmv. On annual basis, only 2.75% of total supplied CH4 to the forest atmosphere was transported to the upper atmosphere via biosphere-atmosphere exchange. Mean CH4 photo-oxidation rate over the forest atmosphere was 3.25 × 10-9 mg cm-3 d-1. Using new and previously published data we present for the first time, a CH4 budget for Sundarbans mangrove ecosystem which in part, revealed the existence of anaerobic CH4 oxidation in the mangrove sediment column. © 2017 Dutta, Bianchi and Mukhopadhyay.