Temporal range: Late Oligocene
Cornwallius is an extinct herbivorous marine mammal of the family Desmostylidae. Cornwallius lived along the North American Pacific Coast from the Early Oligocene (Chattian) through the Oligocene (28.4 mya—20.6 Mya) and existing for approximately.
The type locality is the Chattian Sooke Formation, Vancouver Island, British Columbia, Canada (, paleocoordinates ).
Cornwallius was named by Hay 1923. Its type is Desmostylus sookensis, named by Cornwall 1922 and recombined to Cornwallius sookensis by Hay 1923.
Fossils have been discovered from Baja California Peninsula, Oregon and Washington coasts, and Unalaska Island.