A novel chemolithoautotrophic, hyperthermophilic methanogen was isolated from a submarine hydrothermal system at the Kolbeinsey Ridge, north of Iceland. Based on its 16S rRNA gene sequence, the strain belongs to the order Methanococcales within the genus Methanocaldococcus, with approximately 95 % sequence similarity to Methanocaldococcus jannaschii as its closest relative. Cells of the novel organism stained Gram-negative and appeared as regular to irregular cocci possessing more than 50 polar flagella. These cell appendages mediated not only motility but also adherence to abiotic surfaces and the formation of cell–cell contacts. The new isolate grew at 55–90 °C, with optimum growth at 80 °C. The optimum NaCl concentration for growth was 2.5 % (w/v), and the optimal pH was 6.5. The cells gained their energy exclusively by reduction of CO2 with H2. Selenate, tungstate and yeast extract stimulated growth significantly. The genome size was determined to be in the range 1.8–2.0 kb, and the G+C content of the genomic DNA was 30 mol%. Despite being physiologically nearly identical to the other members of the genus Methanocaldococcus, analysis of whole-cell proteins revealed significant differences. Based on the results from phylogenetic, morphological and protein analyses, we conclude that the novel strain represents a novel species of the genus Methanocaldococcus, for which the name Methanocaldococcus villosus sp. nov. is proposed (type strain KIN24-T80T  = DSM 22612T  = JCM 16315T).