A novel thermophilic, sulfur-oxidizing Gram-negative bacterium, designated strain SS-5^T, was isolated from the Calcite Hot Springs in Yellowstone National Park, USA. The cells were motile rods (1.2–2.8 µm long and 0.6–0.8 µm wide). The new isolate was a facultative heterotroph capable of using elemental sulfur or thiosulfate as an electron donor and O₂ (1–18%; optimum 6%, v/v) as an electron acceptor. Hydrogen did not support growth. The isolate grew autotrophically with CO₂. In addition, strain SS-5^T utilized various organic carbon sources such as yeast extract, tryptone, sugars, amino acids and organic acids. Growth was observed between 55 and 78°C (optimum 70°C; 3.5 h doubling time), pH 6.0 and 8.0 (optimum pH 7.5), and 0 and 0.6%(w/v) NaCl (optimum 0%). The G+C content of the genomic DNA was 32 mol%. Phylogenetic analysis based on the 16S rRNA gene sequence indicated that the isolate was a member of the genus Sulfurihydrogenibium. On the basis of the physiological and molecular characteristics of the new isolate, we propose the name Sulfurihydrogenibium yellowstonense sp. nov. with SS-5^T (=JCM 12773^T=OCM 840^T) as the type strain. In addition, emended descriptions of the genus Sulfurihydrogenibium, Sulfurihydrogenibium subterraneum and Sulfurihydrogenibium azorense are proposed.