A non-motile, rod-shaped and aerobic marine bacterium, designated strain KYW314(T), was isolated from seawater collected from the South Sea, Republic of Korea. Cells were Gram-negative, catalase- and oxidase-positive and had proteolytic activity. The major fatty acids were iso-C(15 : 0) (17.2 %), C(16 : 0) (11.1 %), iso-C(17 : 0) (10.5 %) and C(18 : 1)ω 7c (10.0 %). The DNA G+C content was 56.4 mol%. A phylogenetic tree based on 16S rRNA gene sequences showed that strain KYW314(T) formed a lineage within the genus Pseudidiomarina (95.4-97.3 % sequence similarity) and a distinct branch within the clade containing Pseudidiomarina taiwanensis PIT1(T) and Pseudidiomarina sediminum c121(T). Phenotypic characteristics could distinguish strain KYW314(T) from members of the genus Pseudidiomarina. On the basis of the data presented, strain KYW314(T) represents a novel species, for which the name Pseudidiomarina aestuarii sp. nov. is proposed. The type strain is KYW314(T) (=KCTC 22740(T) =JCM 16344(T)).