A Gram-negative, rod-shaped, psychrophilic, motile, non-spore-forming bacterium, strain U1T, was isolated from Ushuaia located at the southernmost tip of Argentina. On the basis of 16S rRNA gene sequence similarity, strain U1T was found to be closely related to Marinomonas communis (DSM 5604T) and Marinomonas primoryensis (IAM 15010T). At the DNA-DNA level, however, the values for similarity were 41 and 25 %, respectively. The major fatty acids present were iso-C(16 : 0), C(16 : 1)omega7c, iso-C(17 : 1) and C(18 : 1)omega7c and the G+C content of the DNA was 43.6 mol%. All of the above characteristics support the affiliation of strain U1T to the genus Marinomonas. Furthermore, on the basis of phenotypic features, chemotaxonomic characteristics and phylogenetic analysis of the 16S rRNA gene sequence, it appears that strain U1T is distinct from the four Marinomonas species with validly published names. Strain U1T, therefore, represents a novel species, for which the name Marinomonas ushuaiensis sp. nov. is proposed. The type strain of M. ushuaiensis is U1T (=MTCC 6143T=DSM 15871T=JCM 12170T).