A non-cyanobacterial nitrogen-fixing symbiont, 'Candidatus Tectiglobus diatomicola', has been discovered in the tropical North Atlantic, which provides fixed nitrogen to its diatom host in exchange for photosynthetic carbon. This symbiont belongs to the Rhizobiales order, known for nitrogen-fixing symbioses with land plants, and expands the known hosts for this order beyond terrestrial legumes. The study shows that rhizobia–diatom symbioses can contribute as much fixed nitrogen as cyanobacterial nitrogen fixers in the tropical North Atlantic, and may be responsible for nitrogen fixation in vast ocean regions where cyanobacteria are too rare to account for measured rates. Nitrogen is essential for all living organisms and limits life in the ocean. Atmospheric nitrogen gas is the largest reservoir of freely accessible nitrogen, but it is only available to microorganisms that carry the nitrogenase enzyme, which can fix nitrogen into ammonia. Although many marine bacteria and archaea encode nitrogenase, the bulk of oceanic nitrogen fixation has been attributed to cyanobacteria. However, in many ocean regions, cyanobacterial nitrogen fixers are too rare to account for measured rates, suggesting a role for non-cyanobacterial nitrogen fixers. The newly discovered species, 'Candidatus Tectiglobus diatomicola', is a non-cyanobacterial nitrogen fixer that forms a symbiotic relationship with the diatom Haslea. The symbiont provides fixed nitrogen to the host in return for photosynthetic carbon. The genome of 'Candidatus Tectiglobus diatomicola' encodes all genes necessary for nitrogen fixation, and it is likely that the symbiont provides ammonia to the host. The study also shows that the symbiont and host have a coordinated division and vertical transmission of the symbiont, indicating a long-term stable symbiosis. The study also shows that the rhizobia–diatom symbiosis is a major contributor to oceanic nitrogen fixation and has a crucial role in sustaining marine productivity and global carbon sequestration. The findings suggest that non-cyanobacterial nitrogen-fixing symbioses may be more widespread than previously thought, and that the evolution of nitrogen-fixing symbioses in the Rhizobiales order may have occurred independently in marine environments. The discovery of 'Candidatus Tectiglobus diatomicola' expands the known range of diatoms that can associate with nitrogen-fixing symbionts and highlights the importance of these symbioses in the global nitrogen cycle.A non-cyanobacterial nitrogen-fixing symbiont, 'Candidatus Tectiglobus diatomicola', has been discovered in the tropical North Atlantic, which provides fixed nitrogen to its diatom host in exchange for photosynthetic carbon. This symbiont belongs to the Rhizobiales order, known for nitrogen-fixing symbioses with land plants, and expands the known hosts for this order beyond terrestrial legumes. The study shows that rhizobia–diatom symbioses can contribute as much fixed nitrogen as cyanobacterial nitrogen fixers in the tropical North Atlantic, and may be responsible for nitrogen fixation in vast ocean regions where cyanobacteria are too rare to account for measured rates. Nitrogen is essential for all living organisms and limits life in the ocean. Atmospheric nitrogen gas is the largest reservoir of freely accessible nitrogen, but it is only available to microorganisms that carry the nitrogenase enzyme, which can fix nitrogen into ammonia. Although many marine bacteria and archaea encode nitrogenase, the bulk of oceanic nitrogen fixation has been attributed to cyanobacteria. However, in many ocean regions, cyanobacterial nitrogen fixers are too rare to account for measured rates, suggesting a role for non-cyanobacterial nitrogen fixers. The newly discovered species, 'Candidatus Tectiglobus diatomicola', is a non-cyanobacterial nitrogen fixer that forms a symbiotic relationship with the diatom Haslea. The symbiont provides fixed nitrogen to the host in return for photosynthetic carbon. The genome of 'Candidatus Tectiglobus diatomicola' encodes all genes necessary for nitrogen fixation, and it is likely that the symbiont provides ammonia to the host. The study also shows that the symbiont and host have a coordinated division and vertical transmission of the symbiont, indicating a long-term stable symbiosis. The study also shows that the rhizobia–diatom symbiosis is a major contributor to oceanic nitrogen fixation and has a crucial role in sustaining marine productivity and global carbon sequestration. The findings suggest that non-cyanobacterial nitrogen-fixing symbioses may be more widespread than previously thought, and that the evolution of nitrogen-fixing symbioses in the Rhizobiales order may have occurred independently in marine environments. The discovery of 'Candidatus Tectiglobus diatomicola' expands the known range of diatoms that can associate with nitrogen-fixing symbionts and highlights the importance of these symbioses in the global nitrogen cycle.