On the basis of an assigned specimen (USNM 526604, from the Plum Point Member of the Calvert Formation, Early Miocene, Maryland, U.S.A.), Araeodelphis natator Kellogg, 1957, is referred to the Platanistidae. A phylogenetic analysis identifies A. natator as the most stemward member of the family. By contrast, the extant river dolphin, Platanista gangetica (Platanistidae), is one of the most specialized odontocetes. Araeodelphis natator exhibits the following unique combination of characters: overall skull length (condylobasal length) estimated at about 50 cm; rostrum twice the length of the facial region; rostrum wider than deep throughout its entire length; approximately 50 teeth in each quadrant of rostrum; mesorostral canal closed dorsally through anterior half of rostrum by apposition of contralateral premaxillae; cranium with elevated orbits directed anterolaterally; maxillary crest (supraorbital process of frontal and overlapping maxilla) modestly thickened laterally and elevated above midline of skull; non-pneumatized supraorbital eminences; lobe of the pterygoid airsac sinus occupying orbital surface of frontal; zygomatic process compressed transversely; no postglenoid process; and glenoid facet faces medially. Araeodelphis provides new data about the definition and phylogenetic relationships of platanistids with other platanistoids, confirming the sister-group relationship with the extinct squalodelphinids and the ancestral platanistid skull morphology preceding the platanistine-pomatodelphinine split.