A Gram-staining-negative, thin rod-shaped, anaerobic bacterium designated MCWD3T was isolated from sediment of the deep sea in Ulleung Basin, East Sea, Korea. The ranges of temperature, pH and NaCl for growth of this strain were 15– 40°C (optimum 29°C), 5.0–10.0 (optimum pH 6.5), and 1–5%, respectively. The major fatty acids were iso-C15:0 (30%) and iso-C15:0 dimethyl acetal (17%). The major polar lipids were diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, and unidentified aminophospholipids, phospholipids, and aminolipids. The fermentation product from yeast extract was acetate. Phylogenetic analysis based on 16S rRNA genes indicated that the isolate was related to Sporosalibacterium faouarense (92.8% sequence identity), Clostridiisalibacter paucivorans (92.6%), and Brassicibacter mesophilus (92.4%). However, the isolate was differentiated from these genera by both physiological and chemotaxonomical properties. On the basis of a polyphasic taxonomic analysis, we propose that MCWD3T represents a novel taxon with the name Abyssisolibacter fermentans gen. nov. sp. nov.