Three bacterial strains, named h Oe -66 T , h Oe -124 and h Oe -125, were isolated from the haemolymph of different specimens of the flat oyster Ostrea edulis collected in Concarneau bay (Finistère, France). These strains were characterized by a polyphasic approach, including (i) whole genome analyses with 16S rRNA gene sequence alignment and pangenome analysis, determination of the G+C content, average nucleotide identity (ANI), and in silico DNA–DNA hybridization ( is DDH), and (ii) fatty acid methyl ester and other phenotypic analyses. Strains h Oe -66 T , h Oe -124 and h Oe -125 were closely related to both type strains Pseudoalteromonas rhizosphaerae RA15 T and Pseudoalteromonas neustonica PAMC 28425 T with less than 93.3% ANI and 52.3% is DDH values. Regarding their phenotypic traits, the three strains were Gram-negative, 1–2 µm rod-shaped, aerobic, motile and non-spore-forming bacteria. Cells grew optimally at 25 °C in 2.5% NaCl and at 7–8 pH. The most abundant fatty acids were summed feature 3 (C 16:1 ω 7 c /C 16:1 ω 6 c ), C 16:0 and C 17:1 ω 8 c . The strains carried a genome average size of 4.64 Mb and a G+C content of 40.28 mol%. The genetic and phenotypic results suggested that strains h Oe -66 T , h Oe -124 and h Oe -125 belong to a new species of the genus Pseudoalteromonas . In this context, we propose the name Pseudoalteromonas ostreae sp. nov. The type strain is h Oe -66 T (=CECT 30303 T =CIP 111911 T ).