Biogeochemical fluxes and Gibbs energies in sedimentary pore-waters point to the existence of sulfate-reducing ammonium oxidation. This process has not been previously inferred in natural environments. Porewater profiles in the Bay of Bengal (Indian Ocean) demonstrate that significant ammonium disappears at the ammonium-sulfate interface. Loss of ammonium at this horizon greatly exceeds possible nitrogen demand by biomass production. In situ Gibbs energies of reaction (ΔG) in Bay of Bengal and Greenwich Bay (Rhode Island) sediments indicate that sulfate-reducing ammonium oxidation is energy yielding. Relatively small and constant but consistently negative ΔG values for this reaction in both locations match the thermodynamic signature of anaerobic microbial respiration. The ΔG results and the substantial ammonium loss suggest that sulfate-reducing ammonium oxidation occurs in Bay of Bengal sediment. The Greenwich Bay ΔG results suggest that the process may also occur in anoxic sediment where the ammonium concentration profile shows no net loss of ammonium.