A Gram-stain-negative, strictly aerobic and moderately halophilic bacterium, designated strain Gri0909(T), was isolated from a red marine alga, Grinnellia species, in the Yellow Sea of the Republic of Korea. Cells were motile by a single polar flagellum and short-rods showing oxidase and catalase activities. Growth was observed at 10-37 degrees C (optimum, 30 degrees C) and pH 6.0-9.0 (optimum, pH 8.0) and in the presence of 0-12.0 % (w/v) NaCl (optimum, 2.0-3.0 %). Strain Gri0909(T) contained C-16:0, C(18:1 )2-OH, anteisoC(15:0 )and summed feature 8 (comprising C-18:1 omega 7c and/or C-18:1 omega 6c) as major fatty acids. Ubiquinone-10 was identified as the sole isoprenoid quinone. Major polar lipids consisted of phosphatidylethanolamine, an identified phospholipid, four unidentified aminolipids and five unidentified lipids. The G+C content of the genomic DNA calculated from the whole-genome sequence was 59.1 mol%. Strain Gri0909(T) was most closely related to Marivibriohalodurans ZC80(T) with 91.4 % 16S rRNA gene sequence similarity. Phylogenetic analysis based on 16S rRNA gene sequences showed that strain Gri0909(T) formed a phyletic lineage within the family Rhodospirillaceae. The very low 16S rRNA gene sequence similarities, together with distinct phenotypic and chemotaxonomic properties, served to differentiate strain Gri0909(T) from phylogenetically closely related genera. Here it is proposed that strain Gri0909(T) represents a new species of a novel genus of the family Rhodospirillaceae, for which the name Hwanghaeella grinnelliae gen. nov., sp. nov. is proposed. The type strain is Gri0909(T) (=KACC 19793(T)=JCM 32943(T)).