More than 40 specimens of a new endoparasitic ascothoracidan species, described herein as Sessilogoga captiva Kolbasov & Grygier sp. nov. in the family Synagogidae Gruvel, 1905, were found at 35 m depth near Green Island off southeastern Taiwan infecting a colony of the antipatharian Antipathes sp. aff. A. atlantica Gray, 1857. Its sole known congener is S. elongata Grygier, 1990a, an endoparasite of an unidentified antipatharian from Guam. We photographed some specimens in life and used scanning electron microscopy (SEM) and light microscopy to extensively document the fine-scale external morphology of both sexes, including carapace ornamentation (with special attention to lattice organs), the frontal filament complex, the antennular armature, and details of the mouthparts. We also examined the first two naupliar larval stages by SEM. Among the three most plesiomorphic genera of ascothoracidans, the two species of Sessilogoga Grygier, 1990a exhibit several synapomorphies, probably connected with their endoparasitic way of life, that distinguish them from the vagile, possibly micro-predatory species of Synagoga Norman, 1888 and the ectoparasitic, possibly vagile species of Waginella Grygier, 1983a: (i) a reduced frontal filament complex, (ii) absence of epaulets on the sixth thoracomere, (iii) reduction of the setation of the proximal antennular segments, and (iv) more seminal receptacles in any given pair of thoracopods. The new species appears to be gonochoric with females generally larger than males; our find of a juvenile female smaller than any adult male tends to refute the likelihood of protandry.