A Gram-negative, slightly halophilic, strictly aerobic, chemo-organotrophic bacterium was isolated from Mediterranean sea water near Valencia (Spain). 16S rRNA gene sequence comparisons showed that the isolate represented a separate branch within the alpha-3 subclass of the Proteobacteria, now included within the order 'Rhodobacterales'. Jannaschia helgolandensis was the closest relative, but their low sequence similarity and other features indicated that they were not related at the genus level. Isolate 5SM22T produced bacteriochlorophyll a and grew on solid media as regular salmon-pink colonies. Cells are motile rods, with polar flagella. The DNA G+C content is 59.1 mol%. Morphological, physiological and genotypic differences from related, thus far known genera support the description of Thalassobacter stenotrophicus gen. nov., sp. nov. with strain 5SM22T (=CECT 5294T=DSM 16310T) as the type strain.