A Gram-staining-negative, aerobic, non-flagellated, non-gliding and rod-shaped bacterial strain, HD-3(T), was isolated from a tidal flat sediment of Hwangdo in the Yellow Sea, South Korea. Strain HD-3(T) grew optimally at pH 7.0-8.0, at 30 °C and in the presence of 2-3% (w/v) NaCl. Phylogenetic trees based on 16S rRNA gene sequences revealed that strain HD-3(T) joined the cluster comprising the type strains of Jejuia pallidilutea and Hyunsoonleella jejuensis. Strain HD-3(T) exhibited 16S rRNA gene sequence similarity values of 93.8% to each of the type strains of J. pallidilutea and Hyunsoonleella jejuensis. Strain HD-3(T) contained MK-6 as the only menaquinone and iso-C(15:0), iso-C(15:1) G, iso-C(17:0) 3-OH and iso-C(15:0) 3-OH as the major fatty acids. The polar lipid profile of strain HD-3(T), which contained phosphatidylethanolamine and one unidentified lipid as major components and one unidentified phospholipid as a significant component, differed slightly from those of the type strains of J. pallidilutea and Hyunsoonleella jejuensis. The DNA G+C content of strain HD-3(T) was 42.3 mol%. Differential phenotypic and chemotaxonomic properties and phylogenetic data of strain HD-3(T) demonstrated that this strain is distinguishable from J. pallidilutea and Hyunsoonleella jejuensis. On the basis of the data presented, strain HD-3(T) is considered to represent a novel genus and species, for which the name Hwangdonia seohaensis gen. nov., sp. nov. is proposed. The type strain is HD-3(T) ( =KCTC 32177(T) =CCUG 63246(T)).