A Gram-negative, motile and rod-shaped bacterial strain, designated S7-3T, was isolated from a tidal flat sediment at Saemankum on the western coast of Korea. Phylogenetic analyses based on 16S rRNA gene and gyrB sequences showed that strain S7-3T belonged to the genus Shewanella, clustering with Shewanella decolorationis S12T. Strain S7-3T exhibited 98.8 % 16S rRNA gene sequence similarity and 96.8 % gyrB sequence similarity to S. decolorationis S12T, respectively. The 16S rRNA gene sequence similarity values between strain S7-3T and other members of the genus Shewanella were in the range of 93.0–98.0 %. Strain S7-3T contained simultaneously both menaquinones (MK) and ubiquinones (Q); the predominant menaquinone was MK-7 and the predominant ubiquinones were Q-7 and Q-8. The fatty acid profiles of strain S7-3T and S. decolorationis JCM 21555T were similar; major components were C17:1 ω8c, iso-C15:0 and iso-C15:0 2-OH and/or C16:1 ω7c. The DNA G+C content of strain S7-3T was 51.8 mol% and its mean DNA–DNA relatedness value with S. decolorationis JCM 21555T was 43 %. Differential phenotypic properties of strain S7-3T, together with the phylogenetic and genetic distinctiveness, revealed that this strain is distinguishable from recognized Shewanella species. On the basis of the data presented, strain S7-3T is considered to represent a novel Shewanella species, for which the name Shewanella seohaensis sp. nov. is proposed. The type strain is S7-3T (=KCTC 23556T = CCUG 60900T).