The West Indies, well known for its distinctive geological history, is well suited for studies in biogeography and biodiversity. However, the lack of comprehensive studies for much of its unique fauna has hampered insights into their systematics, evolution and phylogeography. The scarcity of data is particularly pronounced for freshwater gastropods in the viviparous family Thiahdae — represented in the archipelago by Hemisinus Swainson, 1840 and Cubaedomus Morrison, 1951. Hitherto described only on the basis of shell morphology, we here redescribe Hemisinus lineolatus (W. Wood, 1828), the type species of Hemisinus, based on type material and museum collections. The anatomy of this species is also described, representing the first such comprehensive study of any Neotropical thiarid. The synonymy of Hemisinus buccinoides Reeve, 1860, with H. lineolatus is confirmed. The species is found to possess a mixture of distinctly thiarid features (e.g., operculum, rachidian, midgut, brood pouch) and features previously undocumented in the family (e.g., gonochohsm with ∼equal sex ratios, glandular palliai oviduct with spermatophore bursa in lateral lamina, spermatophore organ, statoconia). Some of these features (e.g., statoconia, spermatophore organ) are also found in members of the apparent sister group to the Thiaridae — the African-Asian freshwater Paludomidae. Consequently, the combination of features in Hemisinus is consistent with a basal position in the family.