A novel obligately anaerobic, hyperthermophilic, organotrophic archaeon, designated strain 1221nT, was isolated from a hot spring of Uzon Caldera (Kamchatka Peninsula, Russia). Cells of strain 1221nT were non-motile regular cocci, 0.6–1 μm in diameter. The temperature range for growth at pH 6.5 was 65–87 °C, with an optimum at 85 °C. The pH range for growth at 85 °C was 5.5–7.5, with an optimum at pH 6.5. Growth was not observed at or below 6 °C or at or above 90 °C, as well as at or below pH 5.0 and at or above pH 8.0. The isolate fermented a wide range of substrates including proteins: α-keratin, albumin and gelatin. Elemental sulfur was not essential for growth, but stimulated growth. Strain 1221nT synthesized 40 and 120 kDa proteinases localized on the cell envelope. The G+C content of the DNA was 44.4 mol%. Phylogenetic analysis based on 16S rRNA gene sequence comparison indicated that strain 1221nT was affiliated with the genus Desulfurococcus. The level of 16S rRNA gene sequence similarity with other Desulfurococcus species was 96.7–98.1 %, and Desulfurococcus amylolyticus was found to be the most closely related organism. Based on the data from the phylogenetic analysis and the physiological properties of the novel isolate, strain 1221nT should be classified as representing a novel species, for which the name Desulfurococcus kamchatkensis sp. nov. is proposed. The type strain is 1221nT (=DSM 18924T=VKM B-2413T).