A group of five Gram-negative aerobic halophilic bacteria was isolated from the red alga Polysiphonia sp. specimen collected from the Sea of Japan seashore and subjected to a taxonomic study. On the basis of 16S rRNA gene sequence analysis, the novel isolates were affiliated to the genus Labrenzia sharing the highest gene sequence similarities of 98.1-98.4% with the type strain of Labrenzia suaedae KACC 13772T. The DNA-DNA hybridization values of 83-91% obtained between five novel strains, and 26 and 36% between two of the five novel strains and the closest neighbor Labrenzia suaedae KACC 13772T confirmed their assignment to the same separate species. Novel isolates were characterized by Q-10 as the major ubiquinone, by the predominance of C18:1ω7c followed by 11-methyl C18:1ω7c and C14:0 3-ОН in their fatty acid profiles. Polar lipids consisted of phosphatidylcholine, phosphatidylethanolamine, phosphatidylmonomethylethanolamine, phosphatidylglycerol, an unknown aminophospholipid, and an unknown phospholipid. Some of novel strains were found to inhibit growth of Gram-negative and Gram-positive test microorganisms. On the basis of phylogenetic analysis, DNA-DNA hybridization and phenotypic traits, a novel species with the name Labrenzia polysiphoniae sp. nov. (type strain KMM 9699T = rh46T = KACC 19711T), is proposed.