Monostiliferous nemerteans are mostly benthic and free-living, but a few are recognized as commensal and parasitic species. We describe Tetrastemma carneum sp. nov. based on five specimens found from demosponges collected by the use of remotely operated vehicle at a depth of 398 m, off Ofunato, Japan. The new species resembles T. appendiculatum Chernyshev, 1998 described based on specimens discovered from demosponge Esperiopsis digitata (Miklucho-Maclay, 1870) (now accepted as Amphilectus digitatus) trawled at depths of 40–60 m in Peter the Great Bay, in having a pale-colored body with four conspicuously large eyes (approximately 50 µm in the maximum width). However, it is differentiated from T. appendiculatum in possessing a mid-dorsal line on the dorsal surface of the head as well as the number of proboscis nerves. In our molecular phylogenetic tree based on partial sequences of 16S rRNA, cytochrome c oxidase subunit I, 18S rRNA, 28S rRNA, and histone H3 genes, Tetrastemma carneum sp. nov. was nested in the clade of Tetrastemma Ehrenberg, 1828 as a sister taxon to Tetrastemma sp. IP Iturup reported from Peter the Great Bay.