Haplostomides gottoi, new species, is described on the basis of females living in the compound ascidian Polyclinum ?insulsum Sluiter collected in Madagascar (Nossi Mitsio). Among nine known species of the genus (Haplostomatinae), the new species closely resembles H. hibernicus (Scott & Scott, 1895); both species have the same armature formula for legs 1-4. However, H. gottoi is distinguishable from H. hibernicus (H. h.) primarily by the following characters: body much more elongated; antenna with 4 spines (2, in H. h.); maxillule with 3 setae on precoxa and 6 setae on palp (2 and 4, in H. h.); in legs 1-4, terminal and subterminal spines on exopod not fused at base (fused, in H. h.).