A hydrogenotrophic methanoarchaeon, designated strain FWC-SCC4T, was isolated from cold seep sediment of Four-Way Closure Ridge, offshore southwestern Taiwan. Strain FWC-SCC4Tutilizes H2/CO2 or formate, but not acetate, secondary alcohols, methylamines, methanol or ethanol for growth and methane production. Yeast extract is required for growth. The cell morphology is coccoid, with a diameter of 0.8–1.2 µm, and the cell envelope is composed of S-layer protein with Mr about 137.00 kDa. Cells possess multiple flagella and usually occur singly. Strain FWC-SCC4T grows at a temperature range of 20–40 °C (optimum 37 °C) and a pH range of 5.4–7.2 (optimum 7.0). The NaCl range for growth is 0–0.86 M (optimum 0.09 M). The result of phylogenetic analysis of 16S rRNA gene sequences indicates that the most closely related species are Methanoplanus limicola M3T and Methanoplanus endosymbiosus MC1T, with similarities of 95.95 and 95.63%, respectively. The G+C content of the genomic DNA is 40.3 mol%. The overall genome-relatedness indexes (OGRIs) and concatenated ribosomal protein (RBP) phylogenic analysis indicate that strain FWC-SCC4T is a novel lineage of Methanomicrobiaceae. In addition to strain FWC-SCC4T, the differences between Methanomicrobium antiquum MobHT and Methanomicrobium mobile BPT demonstrated by comparative analysis of genomic G+C content and phylogenetic analysis with non-type strain genomes are enough to support the establishment of a novel genus. In conclusion, strain FWC-SCC4T (BCRC AR10058T= NBRC 114595T) is proposed as the type strain of Methanochimaera problematica gen. nov., sp. nov., and Methanoeremita antiquus gen. nov., comb. nov. is proposed as the new name for Methanomicrobium antiquum MobHT (=DSM 21220T= NBRC 104160T).