<jats:p>Strain H2-LR<jats:sup>T</jats:sup>, a 5–18 μm long and 0·7 μm wide filamentous, mesophilic, moderately halophilic, non-motile hydrogenotrophic methanogen, was isolated from marine sediment of Aarhus Bay, Denmark, 1·7 m below the sediment surface. On the basis of 16S rRNA gene comparison with sequences of known methanogens, strain H2-LR<jats:sup>T</jats:sup> could be affiliated to the genus <jats:italic>Methanobacterium</jats:italic>. The strain forms a distinct line of descent within this genus, with <jats:italic>Methanobacterium oryzae</jats:italic> (95·9 % sequence identity) and <jats:italic>Methanobacterium bryantii</jats:italic> (95·7 % sequence identity) as its closest relatives. The 16S rRNA-based affiliation was supported by comparison of the <jats:italic>mcrA</jats:italic> gene, which encodes the α-subunit of methyl-coenzyme M reductase. Strain H2-LR<jats:sup>T</jats:sup> grew only on H<jats:sub>2</jats:sub>/CO<jats:sub>2</jats:sub>. The DNA G+C content is 34·9 mol%. Optimum growth temperature was 45 °C. The strain grew equally well at pH 7·5 and 8. No growth or methane production was observed below pH 5 or above pH 9. Strain H2-LR<jats:sup>T</jats:sup> grew well within an NaCl concentration range of 100 and 900 mM. No growth or methane production was observed at 1 M NaCl. At 50 mM NaCl, growth and methane production were reduced. Based on 16S rRNA gene sequence analysis, the isolate is proposed to represent a novel taxon within the genus <jats:italic>Methanobacterium</jats:italic>, namely <jats:italic>Methanobacterium aarhusense</jats:italic> sp. nov. The type strain is H2-LR<jats:sup>T</jats:sup> (=DSM 15219<jats:sup>T</jats:sup>=ATCC BAA-828<jats:sup>T</jats:sup>).</jats:p>