Stenasodelphis russellae, a new genus and species of extinct dolphin in the odontocete family Pontoporiidae, is based on a partial cranium of Late Miocene age (Tortonian), circa 9 to 10 Ma, from the Little Cove Point Member of the St. Marys Formation, Maryland, USA. Pontoporiidae are amongst the smallest cetaceans, and this dolphin is one of the smallest reported pontoporiids, being probably less than 1.5 m long. It is also the second oldest named pontoporiid in the world. Stenasodelphis russellae shares with late Middle Miocene Brachydelphis mazeasi (eastern South Pacific) and the latest Miocene and Pliocene species of Parapontoporia (North Pacific) left-skew asymmetry of the cranial vertex. Thus, all of the earliest known Pontoporiidae have cranial asymmetry, in contrast to symmetrical crania in the Pliocene pontoporiids, Pontistes rectifrons and Pliopontos littoralis, and the Recent Franciscana, Pontoporia blainvillei. This suggests that cranial asymmetry may be the primitive character state among stem Pontoporiidae. Autapomorphies of Stenasodelphis russellae include small size, thick cranial bones, wide premaxillary sac fossae, a medial projection of each maxilla onto the lateral side of each nasal, and the highest part of the cranial vertex being formed by the nasals rather than the frontals.