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 t (4.4 and 26.8 short tons).
The true value is estimated to fall between these figures, at about 8–10 t (8.8–11.0 short tons).
This size made the sea cow one of the largest mammals of the Holocene epoch, along with whales, and 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 submerge completely.
It had a very thick outer skin, 2.5 cm (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 cm (3–4 in) thick, another adaptation to the frigid climate of the Bering Sea.
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 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 fore limbs were roughly 67 cm (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 about 3.8 cm (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 cm (2 in) long and wide.
In addition to those within its mouth, the sea cow also had stiff bristles 10–12.7 cm (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.

The sea cow's spine is believed to have had seven cervical (neck), 17 thoracic, three lumbar, and 34 caudal (tail) vertebrae.
Its ribs were large, with five of 17 pairs making contact with the sternum; it had no clavicles.
As in all sirenians, the scapula of Steller's sea cow was fan-shaped, being larger on the posterior side and narrower towards the neck.
The anterior border of the scapula was nearly straight, whereas those of modern sirenians are curved.
Like other sirenians, the bones of Steller's sea cow were pachyosteosclerotic, meaning they were both bulky (pachyostotic) and dense (osteosclerotic).
In all collected skeletons of the sea cow, the manus is missing; since Dusisiren—the sister taxon of Hydrodamalis—had reduced phalanges (finger bones), Steller's sea cow possibly did not have a manus at all.

The sea cow's heart was 16 kg (35 lb) in weight; its stomach measured 1.8 m (6 ft) long and 1.5 m (5 ft) wide.
The full length of its intestinal tract was about 151 m (500 ft), equaling more than 20 times the animal's length.
The sea cow had no gallbladder, but did have a wide common bile duct.
Its anus was 10 cm (0.33 ft) in width, with its feces resembling those of horses.
The male's penis was 80 cm (2.6 ft) long.
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 t (4.4 and 26.8 short tons). The true value is estimated to fall between these figures, at about 8–10 t (8.8–11.0 short tons). This size made the sea cow one of the largest mammals of the Holocene epoch, along with whales, and 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 submerge completely. It had a very thick outer skin, 2.5 cm (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 cm (3–4 in) thick, another adaptation to the frigid climate of the Bering Sea. 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 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 fore limbs were roughly 67 cm (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 about 3.8 cm (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 cm (2 in) long and wide. In addition to those within its mouth, the sea cow also had stiff bristles 10–12.7 cm (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.
The sea cow's spine is believed to have had seven cervical (neck), 17 thoracic, three lumbar, and 34 caudal (tail) vertebrae. Its ribs were large, with five of 17 pairs making contact with the sternum; it had no clavicles. As in all sirenians, the scapula of Steller's sea cow was fan-shaped, being larger on the posterior side and narrower towards the neck. The anterior border of the scapula was nearly straight, whereas those of modern sirenians are curved. Like other sirenians, the bones of Steller's sea cow were pachyosteosclerotic, meaning they were both bulky (pachyostotic) and dense (osteosclerotic). In all collected skeletons of the sea cow, the manus is missing; since Dusisiren—the sister taxon of Hydrodamalis—had reduced phalanges (finger bones), Steller's sea cow possibly did not have a manus at all.
The sea cow's heart was 16 kg (35 lb) in weight; its stomach measured 1.8 m (6 ft) long and 1.5 m (5 ft) wide. The full length of its intestinal tract was about 151 m (500 ft), equaling more than 20 times the animal's length. The sea cow had no gallbladder, but did have a wide common bile duct. Its anus was 10 cm (0.33 ft) in width, with its feces resembling those of horses. The male's penis was 80 cm (2.6 ft) long.
Steller's sea cow