A novel obligately anaerobic, extremely thermophilic, organotrophic bacterium, strain ik275marT, was isolated from a Mid-Atlantic Ridge deep-sea hydrothermal vent. Cells were rods surrounded by a sheath-like structure (toga), 0.4–0.9 µm in width and 1.2–6.0 µm in length. Strain ik275marT grew at 37–75 °C, pH 5.6–8.2 and at NaCl concentrations of 10–55 g l−1. Under optimum conditions (70 °C, pH 6.6, NaCl 20 g l−1), doubling time was 32 min. The isolate was able to ferment carbohydrates including starch, cellulose and cellulose derivatives. Acetate, H2 and CO2 were the main products of glucose fermentation. G+C content of DNA was 27 mol%. Phylogenetic analysis of 16S rRNA gene sequences showed that strain ik275marT is a member of the genus Thermosipho. 16S rRNA gene sequence identity with the other species of the genus Thermosipho ranged from 93.7 to 94.5 %. Based on the phylogenetic analysis and physiological properties of the novel isolate, we propose a novel species, Thermosipho affectus sp. nov., with type strain ik275marT ( = DSM 23112T = VKM B-2574T).