A novel strictly anaerobic, hyperthermophilic archaeon, designated strain CDGST, was isolated from a deep-sea hydrothermal vent in the Cayman Trough at 4964 m water depth. The novel isolate is obligate anaerobe and grows chemoorganoheterotrophically with stimulation of growth by sulphur containing compounds. Its growth is optimal at 75 °C, pH 6.0 and under a pressure of 50 MPa. It possesses the broadest hydrostatic pressure range for growth that has ever been described for a microorganism. Its genomic DNA G + C content is 51.11 mol%. The novel isolate belongs to the genus Thermococcus. Phylogenetic analyses indicated that it is most closely related to Thermococcus barossii DSM17882T based on its 16S rRNA gene sequence, and to ‘Thermococcus onnurineus’ NA1 based on its whole genome sequence. The average nucleotide identity scores with these strains are 77.66% for T. barossii and 84.84% for ‘T. onnurineus’, respectively. Based on the draft whole genome sequence and phenotypic characteristics, strain CDGST is suggested to be separated into a novel species within the genus Thermococcus, with proposed name Thermococcus piezophilus (type strain CDGST = ATCC TSD-33T = UBOCC 3296T).