A Gram-staining-negative, aerobic, rod-shaped bacterium, designated strain T202(T), was isolated from the gill of a cultured flounder (Paralichthys olivaceus). Based on 16S rRNA gene sequence similarity, strain T202(T) was a member of the family Colwelliaceae and shared 93.32-96.58 % similarity with type strains of all members of the most closely related genus Thalassomonas. Phylogenetically, the isolate shared a root with the type strains of four marine species, Thalassomonas agariperforans M-M1(T), Thalassomonas agarivorans TMA1(T), Thalassomonas loyana CBMAI 722(T) and Thalassomonas ganghwensis JC2041(T). Optimal growth occurred in the presence of 2-4 % (w/v) NaCl, at pH 7.0-8.0 and at 28 °C. Ubiquinone 8 (Q-8) was the predominant respiratory quinone. The major fatty acids were C16 : 0, summed feature 3 (C16 : 1ω7c and/or C16 : 1ω6c), C16 : 1ω9c and C17 : 1ω8c. The major polar lipids were phosphatidylethanolamine and phosphatidylglycerol. The DNA G+C content of strain T202(T) was 37 mol%. On the basis of polyphasic analysis, especially the phylogenetic relationships and the lower DNA G+C content, strain T202(T) is considered to represent a novel species in a new genus, for which the name Thalassotalea piscium gen. nov., sp. nov. is proposed. The type strain of Thalassotalea piscium is T202(T) ( = JCM 18590(T) = DSM 26287(T) = KCTC 32144(T)). Because Thalassomonas agariperforans M-M1(T), Thalassomonas agarivorans TMA1(T), Thalassomonas loyana CBMAI 722(T) and Thalassomonas ganghwensis JC2041(T) formed a phylogenetic group together with strain T202(T) that was clearly separated from other known strains of Thalassomonas, these four species are reclassified as members of the genus Thalassotalea as Thalassotalea agariperforans comb. nov. (type strain M-M1(T) = KCTC 23343(T) = CCUG 60020(T)), Thalassotalea agarivorans comb. nov. (type strain TMA1(T) = BCRC 17492(T) = JCM 13379(T) = DSM 19706(T)), Thalassotalea loyana comb. nov. (type strain CBMAI 722(T) = LMG 22536(T)) and Thalassotalea ganghwensis comb. nov. (type strain JC2041(T) = IMSNU 14005(T) = KCTC 12041(T) = DSM 15355(T)). The type species of the genus Thalassotalea is Thalassotalea ganghwensis gen. nov., comb. nov. An emended description of the genus Thalassomonas is also proposed.