A mesophilic, slightly halophilic, obligately methylotrophic, methanogenic archaeon, designated strain GTA13T, was isolated from natural gas-bearing confined aquifers in the Minami-Kanto gas field, Japan. The cells were non-motile, slightly irregular cocci, 0.7–1.0 µm in diameter and occurred singly, in pairs or as small aggregates. The cells grew with tri- or dimethylamine but not with H2/CO2, formate, acetate, methanol or dimethyl sulphide. Vitamins, sodium and magnesium were required for growth. Optimal growth occurred at pH 7.0–7.5, 35 °C, 0.35–0.40 M NaCl and 15–50 mM MgCl2. The NaCl range for growth was 0.2–1.3 M. The DNA G+C content was 43.7 mol%. Strain GTA13T showed highest levels of 16S rRNA gene sequence similarity with Methanohalophilus portucalensis FDF-1T (96.4 % sequence similarity) and Methanohalophilus halophilus DSM 3094T (96.0 %). On the basis of physiological and phylogenetic features, strain GTA13T is considered to represent a novel species of the genus Methanohalophilus, for which the name Methanohalophilus levihalophilus sp. nov. is proposed. The type strain is GTA13T ( = NBRC 110099T = DSM 28452T). An emended description of the genus Methanohalophilus is also proposed.