Ninety per cent of marine organic matter burial occurs in continental margin sediments, where a substantial fraction of organic carbon escapes oxidation and enters long-term geologic storage within sedimentary rocks. In such environments, microbial metabolism is limited by the diffusive supply of electron acceptors. One strategy to optimize energy yields in a resource-limited habitat is symbiotic metabolite exchange among microbial associations(1,2). Thermodynamic and geochemical considerations indicate that microbial cometabolisms are likely to play a critical part in sedimentary organic carbon cycling(3-5). Yet only one association, between methanotrophic archaea and sulphate-reducing bacteria, has been demonstrated in marine sedimentsin situ(6,7), and little is known of the role of microbial symbiotic interactions in other sedimentary biogeochemical cycles(8). Here we report in situ molecular and incubation-based evidence for a novel symbiotic consortium between two chemolithotrophic bacteria-anaerobic ammonium-oxidizing (anammox) bacteria and the nitrate-sequestering sulphur-oxidizing Thioploca species-in anoxic sediments of the Soledad basin at the Mexican Pacific margin. A mass balance of benthic solute fluxes and the corresponding nitrogen isotope composition of nitrate and ammonium fluxes indicate that anammox bacteria rely on Thioploca species for the supply of metabolic substrates and account for about 57 +/- 21 per cent of the total benthic N-2 production. We show that Thioploca-anammox symbiosis intensifies benthic fixed nitrogen losses in anoxic sediments, bypassing diffusion-imposed limitations by efficiently coupling the carbon, nitrogen and sulphur cycles.