Local associations between anammox bacteria and obligate aerobic bacteria in the genus Nitrosococcus appear to be significant for ammonia oxidation in oxygen minimum zones. The literature on the genus Nitrosococcus in the Chromatiaceae family of purple sulfur bacteria (Gammaproteobacteria, Chromatiales) contains reports on four described species, Nitrosococcus nitrosus, Nitrosococcus oceani, ‘Nitrosococcus halophilus’ and ‘Nitrosomonas mobilis’, of which only N. nitrosus and N. oceani are validly published names and only N. oceani is omnipresent in the world’s oceans. The species ‘N. halophilus’ with Nc4T as the type strain was proposed in 1990, but the species is not validly published. Phylogenetic analyses of signature genes, growth-physiological studies and an average nucleotide identity analysis between N. oceani ATCC19707T (C-107, Nc9), ‘N. halophilus’ strain Nc4T and Nitrosococcus sp. strain C-113 revealed that a proposal for a new species is warranted. Therefore, the provisional taxonomic assignment Nitrosococcus watsonii is proposed for Nitrosococcus sp. strain C-113T. Sequence analysis of Nitrosococcus haoAB signature genes detected in cultures enriched from Jiaozhou Bay sediments (China) identified only N. oceani-type sequences, suggesting that different patterns of distribution in the environment correlate with speciation in the genus Nitrosococcus.