A Gram-staining-negative, strictly aerobic bacterium, designated SH-1T, was isolated from activated sludge in Korea. Cells were motile rods with a single polar flagellum, showing oxidase-positive and catalase-negative activities. Growth was observed at 25-40 °C (optimum, 37 °C), pH 6.0-9.5 (optimum, pH 7.0) and with 0-0.5 % (w/v) NaCl (optimum, 0 %). Strain SH-1T contained summed feature 3 (comprising C16 : 1ω7c and/or C16 : 1ω6c), C16 : 0, C12 : 0, summed feature 8 (comprising C18 : 1ω7c and/or C18 : 1ω6c) and C10 : 0 3-OH as the major fatty acids and ubiquinone-8 as the sole isoprenoid quinone. Phosphatidylethanolamine was the major polar lipid, and diphosphatidylglycerol, phosphatidylglycerol, one aminophospholipid, one phospholipid, five unidentified aminolipids and two unidentified lipids were also detected as the minor polar lipids. The predominant polyamines were 2-hydroxyputrescine, cadaverine and putrescine. The DNA G+C content was 69.9 mol%. Phylogenetic analysis based on 16S rRNA gene sequences showed that strain SH-1T formed a tight phyletic lineage with Piscinibacter aquaticus IMCC1728T with a 98.3 % sequence similarity. However, the DNA-DNA relatedness value between strain SH-1T and the type strain of P. aquaticus was 38.0±1.8 %. On the basis of phenotypic, chemotaxonomic and molecular properties, it is clear that strain SH-1T represents a novel species of the genus Piscinibacter, for which the name Piscinibacter defluvii sp. nov. is proposed. The type strain is SH-1T (=KACC 18594T=JCM 31230T). An emended description of the genus Piscinibacter is also proposed.