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).　©　2024　The　Authors.