We describe a new species, Tegastes okinawaensis, from 583–711 meter-deep hydrothermal vents in the Okinawa Trough, Japan. This species is clearly distinguishable from its congeners by four combined characters, namely 1) the last three abdominal somites are depressed inwardly towards the body, 2) there is sexual dimorphism in the number of setae of P3 and P4, 3) the male antennule is 9-segmented, with the 7th segment formed by incomplete fusion of two segments, and 4) the caudal rami are small with a distorted quadrangular shape. Additionally, the species is easily distinguishable from its congeners by the distal endopodal segment of P4, which is armed with six setae — this character has never been reported in previous descriptions of congeners. This report is the first of a species in the genus Tegastes living at ocean depths of greater than 580 m in the northwestern Pacific.