Significance Hadal trenches represent the deepest part of the oceans, apparently offering adverse conditions for life, given their high hydrostatic pressure and wide separation from the productive surface waters. However, focusing of reactive organic matter in trench axis sediments potentially promotes complex yet unexplored microbial communities and associated biogeochemical processes. We demonstrate that hadal sediments at depths reaching 10 km are hotbeds for anaerobic nitrogen transformations, dominated by the anammox process, extending the known depth range of this process by ∼6 km. The diversity and phylogeny of identified hadal anammox bacteria suggest that high hydrostatic pressure has not been a major evolutionary driver. Our results provide insights into hadal biogeochemistry and the ability of microbes to adapt to hydrostatic pressure.