A novel anaerobic heterotrophic bacterium, designated strain SWIR-1T , was isolated from a deep-sea hydrothermal vent field sample collected from the Southwest Indian Ridge at a depth of 2700m. Phylogenetic analysis indicated that strain SWIR-1T belongs to the genus Tepidibacter, and the most closely related species are Tepidibacter mesophilus B1T (99.1% 16S rRNA gene sequence similarity), Tepidibacter formicigenes DV1184T (94.6%) and Tepidibacter thalassicus SC562T (93.9%). Strain SWIR 1T shares 77.3–87.2%average nucleotide identity and 21.5–35.7%digital DNA–DNA hybridization values with the three type strains of Tepidibacter species. Cells of strain SWIR-1T were Gram-stain-positive, motile, short straight rods. Endospores were observed in stationary-phase cells when grown on Thermococcales rich medium. Strain SWIR-1T grew at 15–45°C (optimum, 30°C), at pH 5.5–8.0 (optimum, pH 7.0) and with 1.0–6.0% (w/v) NaCl (optimum, 2.0%). Substrates utilized by strain SWIR-1T included complex proteinaceous, chitin, starch, lactose, maltose, fructose, galactose, glucose, rhamnose, arabinose, ribose, alanine, glycine and glycerol. The major fermentation products from glucose were acetate, lactate, H2 and CO2 . Elemental sulphur, sulphate, thiosulphate, sulphite, fumarate, nitrate, nitrite and FeCl3 are not used as terminal electron acceptors. The main cellular fatty acids consisted of iso-C15:0 (28.4%), C15:1 iso F (15.4%) and C16:0 (9.8%). The major polar lipids were phospho lipids and glycolipids. No respiratory quinones were detected. Genomic comparison revealed a distinctive blended gene cluster comprising hyb-tat-hyp genes, which play a crucial role in the synthesis, maturation, activation and export of NiFe-hydrogenase. Based on the phylogenetic analysis, genomic, physiologic and chemotaxonomic characteristics, strain SWIR-1T is considered to represent a novel species within the genus Tepidibacter, for which the name Tepidibacter hydrothermalis sp. nov. is proposed. The type strain is strain SWIR-1T (=DSM 113848T =MCCC 1K07078T )