Proneotherium is an extinct genus of pinniped that lived approximately 20.43 to 15.97 mya[1] during the Early Miocene in what is now Oregon, U.S.

These specimens included cranial and postcranial remains and are all believed to be adult male animals.

[2] Proneotherium was more similar in appearance to modern fur seals and sea lions than walruses.

Autapomorphies of Proneotherium fossils include a continuous, horizontal crest connecting the mastoid and paroccipital processes of the skull.

The hindlimbs of Proneotherium also show the beginnings of evolutionary shifts from a terrestrial lifestyle to a more aquatic one.