The harp seal's head appears somewhat long, wide, and flattened.  The long muzzle tapers slightly, and in adults, can appear upturned.  The eyes are close-set and there is a slight dip to the forehead.  The flippers are relatively small.  The foreflippers are slightly pointed and angular, with a short row of digit endings.  The claws are strong and dark.

The ontogeny of pelage patterns is the most complicated of any pinniped, and in most stages is the species' most distinctive feature.  The newborn's pure white coat, which can be stained yellowish for the first few days by amniotic fluid, persists for about 12 days during which time they are known as “whiecoats.”  At this stage they are difficult to distinguish from other northern phocids born in a similar lanugo pelage.  The “greycoat” and “ragged jacket” stages follow when the underlying juvenile pelage and spotted pattern shows through the lanugo, and then appears as the lanugo is moulted.  From approximately 3- to 4-weeks-old through about age four, harp seals are known as “beaters,” and then as “bedlamers.”  At this stage they have a countershaded coat that is darker bluish gray above, fading to light silver gray below, with an irregular low density of spots and blotches scattered over the entire body.

The adult pattern is complex and varied.  The base color is silvery-white to light gray.  Initially the harp pattern of the adult appears as a faint shadow of what it will become, as the seals transition to the black harp pattern, which consists of a wide and irregular crescentric band with ragged edges dipping down each side before rising up onto the back to fuse over the shoulders.  The head of adult harps is hooded, with the face, chin, upper neck and top of the head being black.  This hood also has ragged margins.  Seen from above, the pattern resembles a large irregular"V” with curving arms.  Black marks may also occur at the insertions of the hindflippers.  Many adults retain dark spots and have incompletely-formed harp patterns on their backs that never fully darken and do not develop a hood.  These animals are known as “spotted harps." 

The dental formula is  I 3/2, C 1/1, PC 5/5. 

Adult males are up to 1.9 m in length and average 135 kg in weight, females up to 1.8 m and 120 kg.  Pups are born at about 85 cm and almost 10 kg.

Harp seals in adult pelage are unlikely to be confused with any other animal.  The silvery-white body, emblazoned with a conspicuous black harp pattern and hood, is unique.  However, the "bedlamer" and "spotted" harp patterns are more generic, and pose some possibility for confusion with the harbor, ringed, gray, hooded, and bearded seals that share their range.  The irregular spots of the bedlamer and spotted harp phases are generally lower density and randomly scattered over the entire body.  Harbor seals tend to have more spots and markings dorsally than ventrally, and are often more strikingly countershaded.  Harp seals lack the rings that characterize ringed seals.  Bearded, hooded and gray seals are all much larger as adults and have distinctive large heads with larger or unique features.  Bearded seals lack spots, have very dense mystacial vibrissae, and have squared-off foreflippers.  Hooded seals have large heads with a wide muzzle at all ages.  Young hooded seals are strikingly countershaded dark, bluish-gray above and lighter gray below, and have no spots.  Adult hooded seals are large with medium to large irregular spots over the entire body, and dark, sooty faces and foreflippers.  Gray seals are also large heavy-looking animals at all ages with proportionately large heads and a large muzzle that is wide and thick, with a robust, often convex bridge.

Harp seals are widespread in the Arctic and North Atlantic oceans and adjacent areas from Hudson Bay and Baffin Island east to Cape Chelyuskin, in northern Russia.  Vagrants reach New England, New York, and northern Europe.  There are regular appearances of large numbers of animals in the coastal waters of central and northern Norway for feeding.  The most famous of the four main breeding aggregations is the "Front," near the Magdalen Islands in the Gulf of St. Lawrence and waters off northeastern Newfoundland and southern Labrador.  Harp seals live chiefly in pack ice, but can be found away from it in summer.