A hyperthermophilic, anaerobic, piezophilic archaeon (strain DY20341T) was isolated from a sediment sample collected from an East Pacific Ocean hydrothermal field (S 1.37° W 102.45°) at a depth of 2737 m. The cells were irregular cocci, 0.8-1.5 µm in diameter. Growth was observed between 50 and 90 °C (opt. 80 °C), pH 5.0 and 8.0 (opt. pH 7.0), 1 and 7 % (w/v) sea salts (Sigma, opt. 3 %), 1 and 4 % (w/v) NaCl (opt. 3 %) and 0.1 and 80 MPa (opt. 30 MPa). The minimum doubling time was 66 min at 30 MPa and 80°C. The isolate was an obligate chemoorganoheterotroph, capable of utilizing complex organic compounds and organic acids including yeast extract, peptone, tryptone, casein, starch, casamino acids, citrate, lactate, acetate, fumarate, propanoate and pyruvate for growth. It was strictly anaerobic and facultatively dependent on elemental sulfur or sulfate as electron acceptors, but did not reduce sulfite, thiosulfate, Fe(III) or nitrate. The presence of elemental sulfur enhanced growth. The G+C content of the genomic DNA was 43.6 ± 1 mol%. 16S rRNA gene sequence analysis revealed that the closest relative of the isolated organism was Palaeococcus ferrophilus DMJT (95.7 % 16S rRNA gene sequence similarity). On the basis of its physiological properties and phylogenetic analyses, the isolate is considered to represent a novel species, for which the name Palaeococcus pacificus sp. nov. is proposed. The type strain is strain DY20341T (= JCM 17873T = DSM 24777T).