A novel Gram-stain-negative, rod-shaped, motile bacterium, designated strain AK21(T), was isolated from coastal surface sea water at Visakhapatnam, India. The strain was positive for oxidase, catalase, lipase, L-proline arylamidase and tyrosine arylamidase activities. The predominant fatty acids were C12:0, C12:0 3-OH, C16:0, C16:1ω9c, C18:1ω9c and summed feature 3 (C16:1ω7c and/or iso-C15:0 2-OH). The polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, one unidentified aminophospholipid, two unidentified phospholipids and one unidentified lipid. Q-10 was the predominant respiratory quinone. The DNA G+C content of the strain was 54.6 mol%. 16S rRNA gene sequence analysis indicated that strain AK21(T) was a member of the genus Marinobacter and was closely related to Marinobacter xestospongiae, with pairwise sequence similarity of 97.2% to the type strain, with similarity to other members of the genus of 94.0-96.8%. The mean DNA-DNA relatedness of strain AK21(T) with M. xestospongiae JCM 17469(T) was 34.5%, and relatedness with Marinobacter mobilis JCM 15154(T) was 40.5%. Phylogenetic analysis showed that strain AK21(T) clustered with the type strains of M. xestospongiae and M. mobilis at distances of 2.9 and 2.8% (97.1 and 97.2% similarity), respectively. Based on the phenotypic characteristics and on phylogenetic inference, it appears that strain AK21(T) represents a novel species of the genus Marinobacter, for which the name Marinobacter nitratireducens sp. nov. is proposed. The type strain of Marinobacter nitratireducens is AK21(T) ( = MTCC 11704(T ) = JCM 18428(T)).