Strain H2-LR^T, 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^T could be affiliated to the genus Methanobacterium. The strain forms a distinct line of descent within this genus, with Methanobacterium oryzae (95·9 % sequence identity) and Methanobacterium bryantii (95·7 % sequence identity) as its closest relatives. The 16S rRNA-based affiliation was supported by comparison of the mcrA gene, which encodes the α-subunit of methyl-coenzyme M reductase. Strain H2-LR^T grew only on H₂/CO₂. 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^T 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 Methanobacterium, namely Methanobacterium aarhusense sp. nov. The type strain is H2-LR^T (=DSM 15219^T=ATCC BAA-828^T).