Costapex baldwinae, new species, is described from deep reef habitats of the southern and eastern Caribbean Sea, including Curaçao, Dominica and Guadeloupe, where it occurs at bathyal depths on sunken wood. It is assigned to the genus Costapex based on phylogenetic analyses using partial sequences of COI, 12S and 16S mitochondrial genes that reveal it to be the sister species of two Indo-Pacific members of this genus. This new species most closely resembles Costapex martinorum (Cernohorsky, 1986) from the Philippines, but differs in being smaller, and in having a slightly lower spire and more prominently beaded spiral sculpture. Of the Caribbean species of costellariids, it is somewhat similar to Nodicostellaria laterculata (Sowerby II, 1874), which occurs in shallower water and has a white or tan rather than dark brown shell, and also a taller spire, more prominent axial sculpture, and a more rounded aperture. It differs from Vexillum styria (Dall, 1889), with which it co-occurs, in having a broader shell with a lower spire, prosocline rather than opisthocline axial ribs, and more prominent, strongly beaded spiral cords. Costapex baldwinae differs from both these taxa in having rachidian teeth with three cusps rather than five cusps (N. laterculata) or seven cusps (V. styria). The genus Costapex was previously known only from Indo-Pacific species. The discovery of this new species represents a significant expansion of the range of this genus into the Caribbean Sea.