<jats:title>Summary</jats:title> <jats:p>A new species of free-living marine nematode, <jats:italic>Deontostoma quadridentatum</jats:italic> sp. nov., is described from a deep-sea sediment sample collected off the South Orkney Islands, the Southern Ocean, during a cruise (KH-19-6-Leg 4) conducted on the R/V <jats:italic>Hakuho-Maru. Deontostoma quadridentatum</jats:italic> sp. nov. differs from its congeners by the possession of: <jats:italic>i</jats:italic>) a cephalic capsule with a smooth posterior edge and without intralobar lacunae; <jats:italic>ii</jats:italic>) interlobar fenestrae with two posterior incisions; <jats:italic>iii</jats:italic>) four odontia on each mandible; <jats:italic>iv</jats:italic>) three onchia; <jats:italic>v</jats:italic>) spicules with velum and lateral process; <jats:italic>vi</jats:italic>) gubernacula with a hollow triangular crus, <jats:italic>vii</jats:italic>) one ventromedian supplement; <jats:italic>viii</jats:italic>) two precloacal subventral rows consisting of 8-10 setae and 12-20 papillae; and <jats:italic>ix</jats:italic>) by the lack of ocelli. Partial 18S rRNA and cytochrome c oxidase subunit I gene sequences were determined for <jats:italic>D. quadridentatum</jats:italic> sp. nov., and the diagnosis of <jats:italic>Deontostoma</jats:italic> and the dichotomous keys to the subfamilies of Leptosomatidae, genera of Deontostomatinae, and species of <jats:italic>Deontostoma</jats:italic> were revised. Two known species, <jats:italic>Triceratonema montredonense</jats:italic> comb. nov. and <jats:italic>Triceratonema papillosum</jats:italic> comb. nov., were transferred from <jats:italic>Deontostoma</jats:italic>. Our molecular phylogenetic analysis did not clarify the position of <jats:italic>Deontostoma quadridentatum</jats:italic> sp. nov., but we placed this species into <jats:italic>Deontostoma</jats:italic> based on morphological observations. We also tried to find a relationship between distribution and morphological characteristic because it seems that there are two phylogenetic lineages that exist in <jats:italic>Deontostoma</jats:italic> based on the distribution of all species. However, no such relationships could be found from our literature survey.</jats:p>