A Gram-stain-negative, strictly aerobic, yellow-pigmented bacterial strain, designated as T28T, was isolated from seawater of the shallow-sea hydrothermal system, Kueishantao Islet, Taiwan, China. Cells were oxidase-negative and catalase-positive rods without gliding motility. Growth was observed at 10-40 °C (optimum, 30 °C), at pH 5.0-8.0 (optimum, pH 6.0) and in the presence of 0-5% (w/v) NaCl (optimum, 2.5%). Strain T28T contained menaquinone 6 as the only isoprenoid quinone. The main cellular fatty acids were iso-C15:0, iso-C15:1 G, and iso-C17:03-OH, summed feature 3 (C16:1ω7c/ω6c). Polar lipids contain diphosphatidylglycerol, phosphatidylethanolamine, two unidentified phospholipids, six unidentified lipids, an unidentified aminolipid, and one unidentified aminophospholipids. The genomic DNA G + C content was 34.4 mol%. The 16S rRNA gene sequence of strain T28T shared highest similarity with Maribacter arcticus (97.7%). Phylogenetic analysis based on 16S rRNA gene sequences revealed that the strain T28T belongs to the genus Maribacter. On the basis of phylogenetic data and several distinct phenotypic characteristics, strain T28T represents a novel species, for which the name Maribacter hydrothermalis sp. nov. is proposed. The type strain is T28T (=CGMCC 1.15788T = JCM 31510T).