The northern fulmar, Arctic fulmar, or simply fulmar (Fulmarus glacialis) is an abundant seabird found primarily in subarctic regions of the North Atlantic and North Pacific oceans. There has been one confirmed sighting in the Southern Hemisphere, with a single bird seen south of New Zealand. Fulmars come in one of two colour morphs; a light one in temperate populations, with white head and body and grey wings and tail, and a dark one in arctic populations, which is uniformly grey; intermediate birds are common. Though similar in appearance to gulls, fulmars are in fact members of the family Procellariidae, which includes petrels and shearwaters.

The northern fulmar and its sister species, the southern fulmar (Fulmarus glacialoides), are the only extant members of the genus Fulmarus. The fulmars are in turn a member of the order Procellariiformes, and they all share certain identifying features. First, they have nasal passages that attach to the upper bill called naricorns; however, nostrils on albatrosses are on the sides of the bill, as opposed to the rest of the order, including fulmars, which have nostrils on top of the upper bill. The bills of Procellariiformes are also unique in that they are split into between seven and nine horny plates. One of these plates makes up the hooked portion of the upper bill, called the maxillary unguis. They produce a stomach oil made up of wax esters and triglycerides that is stored in the proventriculus. This can be sprayed out of their mouths as a defense against predators from a very early age, and as an energy rich food source for chicks and for the adults during their long flights. It will mat the plumage of avian predators, and can lead to their death. Finally, they also have a salt gland that is situated above the nasal passage that helps desalinate their bodies, due to the high amount of ocean water that they imbibe. This gland excretes a high saline solution from their nose.

The northern fulmar was first described as Procellaria glacialis by Carl Linnaeus in 1761, based on a specimen from within the Arctic Circle, on Spitsbergen. The Mallemuk Mountain in Northeastern Greenland is named after the northern fulmar (Danish: Mallemuk).

The northern fulmar was formally described by the Swedish naturalist Carl Linnaeus in 1761 in the second edition of his book Fauna Svecica. He placed it with the other petrels in the genus Procellaria and coined the binomial name Procellaria glacialis. Linnaeus based his description mainly on the "Mallemucke" that had been described and illustrated in 1675 by the German naturalist Friderich Martens in his account of his voyage to Spitzbergen. The northern fulmar is now placed in the genus Fulmarus that was introduced in 1826 by the English naturalist James Stephens. The genus name comes from the Old Norse Fúlmár meaning "foul-mew" or "foul-gull" because of the birds' habit of ejecting a foul-smelling oil. The specific epithet glacialis is Latin for "icy".

Three subspecies are recognised:

The northern fulmar has a wingspan of 102 to 112 cm (40–44 in) and is 46 cm (18 in) in length. Body mass can range from 450 to 1,000 g (16 to 35 oz). This species is grey and white with a pale yellow, thick bill and bluish legs. However, there are both a light morph and dark, or "blue", morph; in the Pacific Ocean there is an intermediate morph as well. Only the dark morph has more than dark edges on the underneath but they all have pale inner primaries on the top of the wings. The Pacific morph has a darker tail than the Atlantic morph.

Like other petrels, their walking ability is limited, but they are strong fliers, with a stiff wing action quite unlike the gulls. They look bull-necked compared to gulls, and have short stubby bills. They are long-lived, with a lifespan of 31 years not uncommon, and exceptionally over 50 years, recorded on Eynhallow in Orkney.

Northern fulmars will feed on shrimps, fish, squid, plankton, jellyfish, and carrion, as well as refuse. When eating fish, they will dive up to 4 metres deep to retrieve their prey.