The characterization of three bacterial strains isolated from cultured oysters and seawater at the Spanish Mediterranean coast has been performed. Strains were phenotypically and genetically characterized and the results led us to identify them as members of the genus Marinomonas. A phylogenetic analysis based on the almost complete 16S rDNA sequences clustered all three strains together (with sequence similarities around 99.8%) in the vicinity of M. communis and M. vaga sequences and distantly related to the other four species of the genus. The most closely related species was M. communis that shared 97.4-97.6% with the Mediterranean strains. DNA-DNA hybridizations were performed to clarify the taxonomic position of our isolates and the results confirmed their specific isolation, with interspecific binding ratios below 59%. We propose the bacteria to constitute a new Marinomonas species, i.e. M. aquamarina and strain 11SM4T (CECT 5080T, CIP 108405T, CCUG 49439T) as the type strain.