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 7.8 million years. The type locality is the Chattian Sooke Formation, Vancouver Island, British Columbia, Canada (48°24′N 123°54′W / 48.4°N 123.9°W, paleocoordinates 48°00′N 115°00′W / 48.0°N 115.0°W). Cornwallius was named by . Its type is Desmostylus sookensis, named by and recombined to Cornwallius sookensis by .