Methane (CH4) oxidation may consume large amounts of dissolved CH4 in lakes and wetlands. Here, we estimated CH4 oxidation rates in an Amazonian floodplain lake during periods representative of the annual variations in water level, using incubations and measurements of δ13C-CH4. A large fraction of the CH4 produced was oxidized (from 34 to ~ 100%, average = 80%), with volumetric CH4 oxidation rates ranging from 1 to 132 mg C m−3 d−1. Heavier values of δ13C-CH4 in surface waters when compared to bottom waters and sediment bubbles corroborates the high CH4 oxidation rates observed. The depth-integrated oxidation rates were of the same magnitude as the calculate CH4 evasion to the atmosphere, which indicates that methane oxidation is an important sink of CH4 and is likely to be important in the many similar lakes encountered in the Amazon River basin.