A Gram-negative, slightly halophilic, strictly aerobic, chemo-organotrophic bacterium was isolated from Mediterranean sea water near Valencia (Spain). Comparison of the almost complete 16S rRNA gene sequence showed that strain 4SM3(T) belonged to the Roseobacter group, with Jannaschia helgolandensis as its closest relative, with a similarity of 98.7 %. DNA-DNA hybridization analysis showed that the Mediterranean isolate had a level of relatedness of less than 42 % with J. helgolandensis and therefore that it represented a novel species of the genus Jannaschia. Phenotypic characteristics gave further evidence that the two organisms are not related at the species level. Isolate 4SM3(T) grows on solid media as irregular pink-red colonies that penetrate into the agar. Cells are rods, motile by a tuft of polar flagella. The DNA base composition is 64.6 mol% G+C. Morphological, physiological and genotypic differences from related species support the description of a novel species, Jannaschia rubra sp. nov., with strain 4SM3(T) (=CECT 5088(T)=DSM 16279(T)) as the type strain.