Harp seals feed primarily on small fish and crustaceans. They are reported to be capable of diving to depths of up to 275 m (900 ft.) and of remaining submerged for up to 15 minutes. Adults average about 2 m (6.5 ft.) in length, with a maximum weight up to about 180 kg (396 lb.). They apparently live for 30 years or more.

At birth, harp seal pups weigh about 11 kg (25 lb.) and are covered with a long, fluffy white fur. For this fur they are called 'whitecoats'. Mothers find their pups among the many whitecoats on the ice floes by scent, and reject all but their own. The pups nurse for about 2 weeks and then fend for themselves. During that two-week period, however, they grow enormously on their mothers' milk, which is rich in butterfat. When they are weaned, they weigh about 40 to 45 kg (88 to 99 lb.).

The adult male is whiteish, with a dark head and a hook-shaped dark mark on its back. Females are usually more mottled with dark and light.

This animal is probably the most commercially important of the seals. Aside from sealers, their principal predators are Greenland sharks, killer whales and polar bears.