We describe a new genus and species of early Oligocene halitheriine dugongid from the Western Atlantic region. Priscosiren atlantica, gen. et sp. nov., differs from all other halitheriine dugongids by displaying the following unique combination of characters: supraorbital processes dorsoventrally thin (≤1 cm) with well-developed prominent posterolateral corner; supraoccipitals wider in the dorsal half than the ventral half; exoccipitals meeting dorsal to the foramen magnum; posttympanic process with a prominent anteroventral process for attachment of m. sternomastoideus; nasals separated in midline; ventral extremity of jugal located ventral to orbit; ventral border of mandible strongly concave; absence of accessory mental foramina; dorsoventrally broad horizontal ramus of mandible; loss of all permanent premolars; and concave anteroventral surface of jugal (a possible autapomorphy). Differs further from the similar species Caribosiren turneri in having lesser rostral deflection (of about 44°); presence of small incisors; lower temporal crests; and slightly larger body size. The relationship of Priscosiren with other dugongids places it close to being a structural as well as temporal ancestor to the Metaxytherium Hydrodamalinae and Dugonginae clades. This supports previous assumptions of a Western Atlantic and Caribbean origin for these groups, and indicates that the halitheriine-dugongine divergence must have occurred no later than the earliest Oligocene. Priscosiren, Caribosiren, and Crenatosiren evidently coexisted in the West Atlantic-Caribbean region, and constitute yet another case of a uniquely patterned fossil sirenian multispecies community.