Northern fur seals belong to the family Otariidae (eared seals), making them different from true seals. Otariids (fur seals and sea lions) have a bear-like head, tightly rolled external ears and can rotate their hind flippers forward, allowing them to walk or run on land using all four flippers.

The most distinctive characteristics of the northern fur seal are its thick, waterproof underfur and its long rear flippers. The underfur is brown and dense (approximately 47,000 hairs/cm2), which keeps the seal dry and warm when in water or on land. The outer pelage (guard hairs) appears black when wet, but on dry animals the color varies between males (black to reddish), females (brown to gray) and pups (black); there can be lighter areas on the throat and chest.

Fur seals swim with their front flippers and can close their nostrils while diving and swimming underwater. The whiskers on the snout (called vibrissae) help the seal’s sense of touch (help detect prey underwater) and change color with age (from black to white).

Northern fur seals have a good sense of hearing and smell; they also have keen eyesight at night and underwater.

Males are much bigger than females. Males weigh between 200 – 250 kg (440 – 550 lbs) and average 2 m (6.6 ft) long. Females weigh up to 45 kg (99 pounds) and average 1.3 m (4.3 ft) long. Newborn pups weigh on average 5.2 kg (11.4 lbs) and 5.7 kg (12.5 lbs) for females and males, respectively.