In many marine environments, a voltage gradient exists across the water–sediment interface resulting from sedimentary microbial activity. Here we show that a fuel cell consisting of an anode embedded in marine sediment and a cathode in overlying seawater can use this voltage gradient to generate electrical power in situ. Fuel cells of this design generated sustained power in a boat basin carved into a salt marsh near Tuckerton, New Jersey, and in the Yaquina Bay Estuary near Newport, Oregon. Retrieval and analysis of the Tuckerton fuel cell indicates that power generation results from at least two anode reactions: oxidation of sediment sulfide (a by-product of microbial oxidation of sedimentary organic carbon) and oxidation of sedimentary organic carbon catalyzed by microorganisms colonizing the anode. These results demonstrate in real marine environments a new form of power generation that uses an immense, renewable energy reservoir (sedimentary organic carbon) and has near-immediate application.