A Gram-staining-negative, catalase- and oxidase-positive, non-motile bacterium, designated strain KOPRI 21160(T), was isolated from Antarctic soil. Based on 16S rRNA gene sequence analysis, strain KOPRI 21160(T) was found to belong to the genus Polaribacter. Sequence similarity between strain KOPRI 21160(T) and the type strains of species of the genus Polaribacter was 94.2-98.3 %. The nearest phylogenetic neighbours of strain KOPRI 21160(T) were Polaribacter butkevichii KCTC 12100(T) (98.3 % similarity) and Polaribacter irgensii KCTC 23136(T) (97.5 %). DNA-DNA relatedness was 50.6 %, between strain KOPRI 21160(T) and P. butkevichii KCTC 12100(T), and 45.2 % between strain KOPRI 21160(T) and P. irgensii KCTC 23136(T). Strain KOPRI 21160(T) grew at 4-37 °C and at pH 7.0-8.5. It could hydrolyse DNA, starch and Tweens 20, 40, 60 and 80. Menaquinone-6 (MK-6) was the only respiratory quinone, and iso-C15 : 0, iso-C15 : 0 3-OH and C15 : 1ω6c were the major cellular fatty acids. The major polar lipids were phosphatidylethanolamine, two unidentified aminolipids and one unidentified lipid. The DNA G+C content was 30.0 mol%. Based on data from our polyphasic study, the organism is considered to represent a novel species of the genus Polaribacter, for which we propose the name Polaribacter sejongensis sp. nov. The type strain is KOPRI 21160(T) ( = KCTC 23670(T) = JCM 18092(T)). Emended descriptions of the genus Polaribacter, Polaribacter butkevichii Nedashkovskaya et al. 2005 and Polaribacter irgensii Gosink et al. 1998 are also proposed.