A novel hyperthermophilic, piezophilic, anaerobic archaeon, designated NCB100T, was isolated from a hydrothermal vent flange fragment collected in the Guaymas basin at the hydrothermal vent site named ‘Rebecca’s Roost’ at a depth of 1997 m. Enrichment and isolation were performed at 100 °C under atmospheric pressure. Cells of strain NCB100T were highly motile, irregular cocci with a diameter of ~1 µm. Growth was recorded at temperatures between 70 and 112 °C (optimum 105 °C) and hydrostatic pressures of 0.1–80 MPa (optimum 40–50 MPa). Growth was observed at pH 3.5–8.5 (optimum pH 7) and with 1.5–7 % NaCl (optimum at 2.5–3 %). Strain NCB100T was a strictly anaerobic chemo-organoheterotroph and grew on complex proteinaceous substrates such as yeast extract, peptone and tryptone, as well as on glycogen and starch. Elemental sulfur was required for growth and was reduced to hydrogen sulfide. The fermentation products from complex proteinaceous substrates were CO2 and H2. The G+C content of the genomic DNA was 41.3 %. Phylogenetic analysis of the 16S rRNA gene sequence revealed that strain NCB100T belongs to the genus Pyrococcus, showing 99 % similarity with the other described species of the genus Pyrococcus. On the basis of physiological characteristics, DNA G+C content, similarity level between ribosomal proteins and an average nucleotide identity value of 79 %, strain NCB100T represents a novel species for which the name Pyrococcus kukulkanii sp. nov. is proposed. The type strain is NCB100T (=DSM 101590T=Souchothèque de Bretagne BG1337T).