During several excavations in Slovakia at the base of the Male Karpaty Mountains (near the junction of the Morava and Danube Rivers), two skulls and numerous mandibular, dental and postcranial bones of early mid-Miocene (16.3–12.8 Ma) seals were collected. Isolated mandibles and many individual teeth were found at this locality, with some teeth in situ corresponding morphologically with the mandible and skull of Devinophoca claytoni, and others perfectly associating to the mandible and skull of the recently described Devinophoca emryi. Based on this material, two species of the previously unknown phocid subfamily Devinophocinae Koretsky et Holec, 2002 have been described. However, the mandibular morphology of the type species Devinophoca claytoni has remained unknown. Here, we present a first description of the mandible of this species. Morphological assessments reveal that the D. claytoni mandible has posterior alveoli larger than anterior; flat mandibular body low in height; alveoli of p4 larger than m1; and a unique devinophocine combination of incisors (I3/1) that differs from those in the extant subfamilies Cystophorinae (I2/1), Monachinae (I2/2) and Phocinae (I3/2).