A Gram-stain-negative, strictly aerobic, non-motile bacterium, designated strain S2-CT, was isolated from an estuary sediment in South Korea and subjected to a polyphasic taxonomic investigation. Cells were catalase- and oxidase-positive rods without gliding motility. Growth of strain S2-CT was observed at 15-40 °C (optimum, 30 °C), at pH 5.5-9.0 (optimum, pH 6.5-7.5) and in the presence of 0-7.0 % (w/v) NaCl (optimum, 0-2 %). Only ubiquinone-8 (Q-8) was detected as the isoprenoid quinone and iso-C16 : 0, iso-C15 : 0, summed feature 9 (comprising iso-C17 : 1ω9c and/or C16 : 0 10-methyl), iso-C11 : 0, iso-C11 : 0 3-OH and iso-C14 : 0 were found to be the major cellular fatty acids. Strain S2-CT contained phosphatidylethanolamine, phosphatidylglycerol and diphosphatidylglycerol as the major polar lipids. The G+C content of the genomic DNA was 63.8 mol%. Phylogenetic analysis, based on 16S rRNA gene sequences, revealed that strain S2-CT formed a distinct phyletic lineage within the genus Lysobacter. Strain S2-CT was most closely related to Lysobacter daejeonensis GH1-9T with 97.1 % 16S rRNA gene sequence similarity and the DNA-DNA relatedness value between strain S2-CT and the type strain of L. daejeonensis was 45.7 ± 2.2 %. On the basis of the phenotypic, chemotaxonomic and molecular features, strain S2-CT clearly represents a novel species of the genus Lysobacter, for which the name Lysobacter aestuarii sp. nov. is proposed. The type strain is S2-CT ( = KACC 18502T = JCM 31130T).