A mesophilic, non-motile, hydrogenotrophic, rod-shaped methanogen, designated M2^T, was isolated from Holocene permafrost sediments of the Kolyma lowland in the Russian Arctic. Cells were 3–6 μm long and 0.45–0.5 μm wide. Strain M2^T grew on H₂/CO₂ and formate. Optimum conditions for growth were 37 °C, pH 6.8–7.2 and 0.1 M NaCl. The DNA G+C content was 38.1 mol%. On the basis of 16S rRNA gene sequence comparison with known methanogens, strain M2^T was affiliated with the genus Methanobacterium and was most closely related to Methanobacterium veterum MK4^T and Methanobacterium bryantii DSM 863^T (both 99 % 16S rRNA gene sequence similarity). However, no significant DNA–DNA relatedness was observed between strain M2^T and these type strains. We propose that strain M2^T represents a novel species, with the name Methanobacterium arcticum sp. nov., with type strain M2^T (=DSM 19844^T =VKM B-2371^T).