We describe five new specimens of Hippopotamidae from the Miocene of Napudet, a new site in southwestern Turkana Basin, Kenya. These specimens include fragmentary maxillae with teeth and a well-preserved mandibular symphysis. We attribute them to Kenyapotamus ternani, the least known species within Kenyapotamus, on the basis of relatively small dental dimensions and a clear distinction between the parastyle and the mesiostyle on the upper molars. This attribution suggests an age older than 10 Ma for Napudet. A cladistic analysis integrating these new data makes it possible to evaluate the relationships between middle Miocene hippopotamids and later representatives. The mandibular symphysis from Napudet, defining the plesiomorphic condition for mandibular morphology in Hippopotaminae, could be crucial for future phylogenetic reconstructions of the family. Mandibular morphology is a fast-evolving complex of characters, key in reconstructing the behavior and the past diversity of Hippopotamidae. Finally, this material demonstrates the potential of Napudet for augmenting the fossil record for a relatively ill-documented time interval (13–10 Ma).