A Gram-negative, rod-shaped, motile and aerobic bacterium, designated strain HJ51(T), was isolated from a seawater sample from the East Sea, near South Korea. The isolate grew slowly at 4 °C, was able to grow at 40 °C, required NaCl and grew optimally at pH 6.5-7.0. The G+C content of the genomic DNA was 41.8 mol%. The major fatty acids were summed feature 4 (C(16 : 1)ω7c and/or iso-C(15 : 0) 2-OH), C(16 : 0) and summed feature 7 (C(18 : 1)ω7c, C(18 : 1)ω9t and/or C(18 : 1)ω12t). Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain HJ51(T) belonged to the genus Pseudoalteromonas and had 91.7-98.9 % 16S rRNA gene sequence similarity with type strains of species of the genus Pseudoalteromonas. Strain HJ51(T) had 7.2 % DNA-DNA relatedness with Pseudoalteromonas mariniglutinosa DSM 15203(T) and 12.9 % with Pseudoalteromonas prydzensis DSM 14232(T). On the basis of the phenotypic, phylogenetic and genomic data, strain HJ51(T) represents a novel species of the genus Pseudoalteromonas, for which the name Pseudoalteromonas donghaensis sp. nov. is proposed. The type strain is HJ51(T) (=KCTC 22219(T)=LMG 24469(T)).