Here we describe a new taxon of late Oligocene dugongine from the Western Atlantic and Caribbean region. Known from cranial and postcranial material, Callistosiren boriquensis, gen. et sp. nov., differs from other members of the group by displaying the following unique combination of characters: nasal process of premaxilla tapering but thickened; supraorbital process of frontal dorsoventrally thin (<2 cm) and well developed with recurved, bluntly pointed posterolateral corner; ventral rim of orbit not overhanging the lateral surface of the jugal; pterygoid fossa tall; tusks large, lozenge- or kiteshaped in cross-section, and with enamel confined to the medial side; lacrimals with thin, elongated dorsal process that is wedged between the premaxilla and frontal; and osteosclerotic but minimally pachyostotic ribs and vertebrae. A phylogenetic analysis places Callistosiren as one of the more basal members of the Dugonginae, but we note that the relationships and taxonomy of more derived members are in need of revision. Characters of the cranial morphology, such as the shape and size of the incisor tusks and the nature of the contact between the premaxilla and frontal, indicate that this new taxon may have fed preferentially on large species of seagrasses and their rhizomes. In addition, possession of osteosclerotic but minimally pachyostotic ribs and vertebrae is a unique feature unknown in other extinct dugongines and may indicate foraging behavior not known from other fossil sirenians.