The cerithioid Supplanaxis nucleus (Bruguière, 1789) is widespread in the Caribbean, where it lives in often dense aggregates on hard surfaces in the middle-high intertidal. Molecular evidence shows that it comprises two species that are in fact morphologically diagnosable. We fix the nomenclature of Supplanaxis nucleus by designating a sequenced neotype from Bruguière's historical locality of Barbados, and identify the second, cryptic species as S. nancyae (Petuch, 2013). The two live syntopically across the Caribbean and form a closely related species group with the Panamic S. planicostatus (G.B. Sowerby I, 1825). Planaxis nucleola Mörch, 1876, described from St Croix, in the Virgin Islands, never again recorded in the literature but listed as a synonym of S. nucleus in taxonomic authority lists, is recognized as a valid species of Hinea Gray, 1847. Proplanaxis Thiele, 1929 and Supplanaxis Thiele, 1929, are synonyms and the latter is given precedence over the former.