Two orange, rod-shaped, Gram-reaction-negative, aerobic bacterial strains devoid of flagella and gliding motility, designated strains KYW371(T) and KS18 were isolated from a seawater sample and a shellfish Ruditapes philippinarum, respectively, collected from Gwangyang Bay, Republic of Korea. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the two strains belonged to the family Flavobacteriaceae; and that strain KYW371(T) was most closely related to Algibacter mikhailovii LMG 23988(T) (96.7 % sequence similarity), Pontirhabdus pectinivorans JC2675(T) (96.3 %), Postechiella marina M091(T) (95.6 %) and Hyunsoonleella jejuensis CNU004(T) (95.3 %). The 16S rRNA gene sequence similarity (98.8 %) and DNA-DNA relatedness (78.1 %) between strains KYW371(T) and KS18 indicated that these two strains represented a single species. The predominant cellular fatty acids of strain KYW371(T) were iso-C15 : 1 G, iso-C15 : 0, iso-C15 : 0 3-OH and iso-C17 : 0 3-OH. Flexirubin-type pigments were absent. MK-6 was the only isoprenoid quinone and the DNA G+C content was 34.8-36.6 mol%. Data from this taxonomic study employing a polyphasic approach suggested that the isolates represent a novel species in a new genus in the family Flavobacteriaceae, for which the name Marinivirga aestuarii gen. nov., sp. nov. is proposed. The type strain is KYW371(T) ( = KCTC 23449(T) = JCM 17452(T)), and an additional strain of the species is KS18 ( = KCTC 23128 = JCM 16845). Emended descriptions of the genera Hyunsoonleella, Jejuia and Pontirhabdus and the species Hyunsoonleella jejuensis, Jejuia pallidilutea and Pontirhabdus pectinivorans are also proposed.