The Steller Sea Cow was very large. It weighted from 5400 to 11196 kilograms. Georg Steller described the sea cow as having a length of 296 inches or 24 2/3 feet. The one he measured was a female. Steller described the head and neck to be short and weak. The nostrils were in pairs and were located at the tip of the snout. The eyes were described as small for its size. Adult sea cows did not have teeth. They had many  vibrissae-like bristles that were used as teeth. The sea cow had an unusual thick black layer of skin. This layer of skin may have protected it against rocks in the shallow water were it fed.

The Steller Sea Cow was a herbivore, meaning it only ate plants such as soft kelps and brown algae. Sea grass beds were an important feeding ground. Sea cows had to eat about 100 pounds of plants a day to survive. The upper lip of the sea cow was spilt down the middle  to allow it to grab plants. Steller described the sea cow's losing enough weight during the winter that their ribs were visible.