A novel anaerobic, alkaliphilic, Gram-positive staining bacterium was isolated from a hydrothermal chimney in the Prony Bay, New Caledonia. This strain designated FatMR1T grew at temperatures from 20 to 55 °C (optimum 37 °C) and at pH between 7.5 and 10.5 (optimum 8.8–9). NaCl is not required for growth (optimum 0.2–0.5 %), but is tolerated up to 3 %. Sulfate, thiosulfate, elemental sulfur, sulfite, nitrate and nitrite are not used as terminal electron acceptors. Strain FatMR1T fermented pyruvate, yeast extract, peptone and biotrypcase and used fructose as the only sugar. The main fermentation products from fructose and proteinaceous compounds (e.g. peptone and biotrypcase) were acetate, H2 and CO2. Crotonate was disproportionated to acetate and butyrate. The predominant cellular fatty acids were C14:0 and C16:0. The G + C content of the genomic DNA was 37.1 mol %. On the basis of phylogenetic, genetic, and physiological properties, strain FatMR1T (=DSM 25890T, =JCM 18390T) belonging to the phylum Firmicutes, class Clostridia, order Clostridiales, is proposed as a novel species of the genus Alkaliphilus, A. hydrothermalis sp. nov.