A taxonomic study was carried out on a novel bacterial strain, designated AM1R11(T), which was isolated from seawater of Jeju Island in Korea. Cells of the isolate were found to be Gram-negative, rod-shaped and non-motile. Comparison of the 16S rRNA gene sequences indicated that the isolate belonged to the family Cytophagaceae, with Dyadobacter ginsengisoli Gsoil 043(T) as its closest relative, with a similarity of 96.6 %. It contained summed feature 3 (comprising C16 : 1ω7c and/or C16 : 1ω6c, 36.9 %), iso-C15 : 0 (16.5 %) and C16 : 1ω5c (16.3 %) as the major fatty acids and MK-7 as the predominant menaquinone. The polar lipid profile of strain AM1R11(T) revealed the presence of phosphatidylethanolamine, one aminolipid and four unidentified lipids (L1, L2, L3 and L4). The DNA G+C content of strain AM1R11(T) was 45.1 mol%. On the basis of the evidence presented, it is concluded that strain AM1R11(T) represents a novel species of the genus Dyadobacter, for which the name Dyadobacter jejuensis sp. nov. is proposed. The type strain is AM1R11(T) ( = KACC 16446(T) = JCM 17918(T)).