﻿AbstractMonostiliferous nemerteans in the genusTetrastemmaEhrenberg, 1828 are generally characterized as having four eyes, and they occur worldwide, from the intertidal zone to the deep-sea bottom. Recent extensive sampling ofTetrastemmahas explored the high species diversity, including many undescribed forms, but phylogenic analysis has revealed non-monophyly of the genus. We herein describe three new species of the genus (T.albumsp. nov.,T.personasp. nov., andT.shohoensesp. nov.) from northwestern Pacific waters based on specimens collected by dredging or by use of a remotely operated vehicle at depths of 116–455 m. Since anatomical and histological characters traditionally used in systematics of the genus are sometimes interspecifically uniform, a histology-free approach is applied for the species descriptions in this study. To confirm the generic affiliation of the new species, a molecular phylogenetic analysis based on partial sequences of cytochromecoxidase subunit I,16SrRNA,18SrRNA,28SrRNA, and histoneH3genes was performed. Our result shows that all three new species are nested in a subclade formed by species from the North Pacific and American Atlantic, inferring that geographic distribution does not reflect the cladogenesis ofTetrastemma. Furthermore, twoTetrastemmaspecies with a cylindrical stylet basis,T.freyaeChernyshev et al., 2020from off the coast of India and Hawaii andT.shohoensesp. nov.from Shoho Seamount, Japan, constitute a clade in the resulting tree.