A Gram-negative, non-motile and rod-shaped bacterial strain, designated RA2-3(T), was isolated from a sea squirt (Halocynthia roretzi) collected from the South Sea, Korea, and its taxonomic position was investigated by using a polyphasic approach. Strain RA2-3(T) was observed to grow optimally at 25 °C, at pH 7.0-7.5 and in the presence of 2 % (w/v) NaCl. Strain RA2-3(T) exhibited the highest 16S rRNA gene sequence similarity values to the type strains of Litoreibacter meonggei (95.7 %), Planktotalea frisia (95.6 %), Thalassobius gelatinovorus (95.5 %) and Pelagicola litoralis (95.4 %). A neighbour-joining phylogenetic tree based on 16S rRNA gene sequences revealed that strain RA2-3(T) clustered with the type strains of Planktotalea frisia, Pelagicola litoralis, Pacificibacter maritimus and Roseovarius marinus. Strain RA2-3(T) was found to contain Q-10 as the predominant ubiquinone and C18:1 ω7c as the major fatty acid. The major polar lipids detected in strain RA2-3(T) were phosphatidylcholine, phosphatidylglycerol, one unidentified aminolipid and one unidentified lipid. The DNA G+C content of strain RA2-3(T) was 52.9 mol%. On the basis of the phylogenetic, chemotaxonomic and phenotypic properties, strain RA2-3(T) is considered to represent a new genus and species within the family Rhodobacteraceae, for which the name Halocynthiibacter namhaensis gen. nov., sp. nov. is proposed. The type strain of H. namhaensis is RA2-3(T) (=KCTC 32362(T)=NBRC 109999(T)).