Four heterotrophic, antimicrobial, motile, marine bacterial strains, 27-4T, 8-1, M6-4.2 and S26, were isolated from aquaculture units in Spain, Denmark and Greece. All four strains produced the antibiotic compound tropodithietic acid, which is a key molecule in their antagonism against fish pathogenic bacteria. Cells of the strains were Gram-reaction-negative, rod-shaped and formed star-shaped aggregates in liquid culture and brown-coloured colonies on marine agar. The predominant cellular fatty acids were C18 : 1ω7c, C16 : 0, C11 methyl C18 : 1ω7c and C16 : 0 2-OH, and the polar lipids comprised phosphatidylglycerol, diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylcholine, an aminolipid, a phospholipid and an unidentified lipid. The strains grew optimally at 31-33 °C. Growth was observed at a salt concentration between 0.5 and 5-6 % NaCl with an optimum at 2-3 %. The pH range for growth of the strains was from pH 6 to 8-8.5 with an optimum at pH 7. Based on 16S rRNA gene sequence analysis, the strains are affiliated with the genus Phaeobacter. The genome sequences of the strains have a DNA G+C content of 60.1 % and share an average nucleotide identity (ANI) of more than 95 %. The four strains are distinct from the type strains of the closely related species Phaeobactergallaeciensis and Phaeobacterinhibens based on an ANI of 90.5-91.7 and 89.6-90.4 %, respectively, and an in silico DNA-DNA hybridization relatedness of 43.9-46.9 and 39.8-41.9 %, respectively. On the basis of phylogenetic analyses as well as phenotypic and chemotaxonomic properties, the isolates are considered to represent a novel species, for which the name Phaeobacter piscinae sp. nov. is proposed. The type strain is 27-4T (=DSM 103509T=LMG 29708T).