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The methanogenic strain Mx-05^T was isolated from the human fecal microbiome. A phylogenetic analysis based on the 16S rRNA gene and protein marker genes indicated that the strain is affiliated with the order <i>Methanomassiliicoccales</i>. It shares 86.9% 16S rRNA gene sequence identity with <i>Methanomassiliicoccus luminyensis</i>, the only member of this order previously isolated. The cells of Mx-05^T were non-motile cocci, with a diameter range of 0.4–0.7 μm. They grew anaerobically and reduced methanol, monomethylamine, dimethylamine, and trimethylamine into methane, using H₂ as an electron donor. H₂/CO₂, formate, ethanol, and acetate were not used as energy sources. The growth of Mx-05^T required an unknown medium factor(s) provided by <i>Eggerthella lenta</i> and present in rumen fluid. Mx-05^T grew between 30 °C and 40 °C (optimum 37 °C), over a pH range of 6.9–8.3 (optimum pH 7.5), and between 0.02 and 0.34 mol.L⁻¹ NaCl (optimum 0.12 mol.L⁻¹ NaCl). The genome is 1.67 Mbp with a G+C content of 55.5 mol%. Genome sequence annotation confirmed the absence of the methyl branch of the H₄MPT Wood–Ljungdahl pathway, as described for other <i>Methanomassiliicoccales</i> members. Based on an average nucleotide identity analysis, we propose strain Mx-05^T as being a novel representative of the order <i>Methanomassiliicoccales</i>, within the novel family <i>Methanomethylophilaceae</i>, for which the name <i>Methanomethylophilus alvi</i> gen. nov, sp. nov. is proposed. The type strain is Mx-05^T (JCM 31474T).