Narwhals lack a dorsal fin, and in adults the flippers are turned upward at the tips. Their mottled gray bodies are darker above than below, and they usually attain a length of 3.5 to 5 metres (11.5 to 16.4 feet), with males being larger than females. Adult males weigh about 1,600 kg (3,500 pounds); females weigh about 1,000 kg.

The narwhal has two teeth, both at the tip of the upper jaw, but usually only the left tooth develops. The resulting tusk grows to more than three metres and is grooved on the surface in a left-handed spiral. The undeveloped right tooth in males and usually both teeth in females remain vestigial. On rare occasions two tusks may develop in females as well as males. Although a variety of theories have been proposed for the specific function of this tooth, recent observations suggest that the males use their tusks in aggression against one another as they compete for mates. The tusk was prized in medieval times as the horn of the fabled unicorn.

Narwhals are usually found in groups of 15 to 20, but herds of several thousand have been seen. Newborn narwhal calves are about 1.6 metres long. Calves are weaned after a year or more; females reach sexual maturity at about six years and males at eight. They feed on fish, squid, and shrimp.

Predators of the narwhal include killer whales and, to a lesser extent, polar bears and walruses. Eskimos () hunt them mainly for the ivory tusk and the skin, which is rich in vitamin C. Occasionally, hundreds of narwhals and beluga whales (beluga) become trapped by pack ice in a pool of open water (savssat in the West Greenland dialect). The whales may then become prey to the local Inuit hunters or die as ice closes the hole.

Narwhals and belugas are related, and together they compose the family Monodontidae of suborder Odontoceti (the toothed whales). The origin of the term narwhal may be the Icelandic words nar, meaning “corpse” (in reference to its pale colour), and hvalr (whale). The scientific name is derived from the Greek words for “single-toothed” and “single-horned,” respectively.