It was a part of the order Sirenia and a member of the family Dugongidae, of which its closest living relative, the 3-meter (9.8 ft) long dugong (Dugong dugon), is the sole surviving member. It had a thicker layer of blubber than other members of the order, an adaptation to the cold waters of its environment. Its tail was forked, like that of cetaceans. Lacking true teeth, it had an array of white bristles on its upper lip and two keratinous plates within its mouth for chewing. It fed mainly on kelp and communicated with sighs and snorting sounds. Evidence suggests it was a monogamous and social animal living in small family groups and raising its young, similar to extant sirenians.
Steller's sea cow was named after Georg Wilhelm Steller, a naturalist who discovered the species in 1741 on Vitus Bering's Great Northern Expedition when the crew became shipwrecked on Bering Island. Much of what is known about its behavior comes from Steller's observations on the island, documented in his posthumous publication On the Beasts of the Sea. Within twenty-seven years of discovery by Europeans, the slow-moving and easily caught mammal was hunted into extinction for its meat, fat, and hide.
Steller's sea cows grew to be 8 to 9 m (26 to 30 ft) long as adults, much larger than extant sirenians. Georg Steller's writings contain two contradictory estimates of weight: 4 and 24.3 metric tons (4.4 and 26.8 short tons). The true value is estimated to fall between these figures, at about 8–10 metric tons (8.8–11.0 short tons). This size made the sea cow one of the largest mammals of the Holocene epoch, along with whales. The sea cow's large size was likely an adaptation to reduce its surface-area-to-volume ratio and conserve heat. Unlike other sirenians, Steller's sea cow was positively buoyant, meaning that it was unable to completely submerge. It had a very thick outer skin, 2.5 centimeters (1 in), to prevent injury from sharp rocks and ice and possibly to prevent unsubmerged skin from drying out. The sea cow's blubber was 8–10 centimeters (3–4 in) thick, another adaptation to the frigid climate of the Bering Sea, where it lived. Its skin was brownish-black, with white patches on some individuals. It was smooth along its back and rough on its sides, with crater-like depressions most likely caused by parasites. This rough texture led to the animal's being nicknamed the "bark animal". Hair on its body was sparse, but the insides of the sea cow's flippers were covered in bristles. The forelimbs were roughly 67 centimeters (26 in) long, and the tail fluke was forked.
The sea cow's head was small and short in comparison to its huge body. The animal's upper lip was large and broad, extending so far beyond the lower jaw that the mouth appeared to be located underneath the skull. Unlike other sirenians, Steller's sea cow was toothless and instead had a dense array of interlacing white bristles on its upper lip. The bristles were approximately 3.8 centimeters (1.5 in) in length and were used to tear seaweed stalks and hold food. The sea cow also had two keratinous plates located on its palate and mandible, used for chewing. According to Steller, these plates (or "masticatory pads") were held together by interdental papillae, a part of the gums, and had many small holes containing nerves and arteries.
As with all sirenians, the sea cow's snout pointed downwards, which allowed it to better grasp kelp. The sea cow's nostrils were roughly 5 centimeters (2 in) long and wide. In addition to those within its mouth, the sea cow also had stiff bristles 10–12.7 centimeters (3.9–5.0 in) long protruding from its muzzle. Steller's sea cow had small eyes located halfway between its nostrils and ears with black irises, livid eyeballs, and canthi which were not externally visible. The animal had no eyelashes, but like other diving creatures such as sea otters, Steller's sea cow had a nictitating membrane, which covered its eyes to prevent injury while feeding. The tongue was small and remained in the back of the mouth, unable to reach the masticatory (chewing) pads.