The female of Haplostoma banyulensis (Brément, 1909) is redescribed on the basis of specimens parasitizing the compound ascidian Trididemnum tenerum (Verrill) at Strangford Lough, Northern Ireland. It is shown to have a mandible, which was not reported in the original description or in succeeding studies. This species can be distinguished from its 10 congeners of subgroup 1 of Haplostoma by the following characteristics: labrum with six distinct processes subequally spaced; legs 1–4 exopods with armature formula 5,4,5,5; leg 4 with longest exopod; copulatory organs including two copulatory pores and two seminal receptacles; and subcylindrical caudal ramus with three spinal elements on terminal margin.