The northern harrier (Circus hudsonius), also known as the marsh hawk or ring-tailed hawk, is a bird of prey. It breeds throughout the northern parts of the Northern Hemisphere in Canada and the northernmost United States.

The northern harrier migrates south in winter, with breeding birds in Canada and northern Great Plains of the U.S. moving to the American south, Mexico, and Central America. In the midwestern, mountain west, and north Atlantic states of the U.S., they may be present all year. This bird inhabits prairies, open areas, and marshes.

In 1750 the English naturalist George Edwards included an illustration and a description of the northern harrier in the third volume of his A Natural History of Uncommon Birds. He used the English name "The Ring-tail'd Hawk". Edwards based his hand-coloured etching on a bird collected near the Hudson Bay in Canada and brought to London by James Isham. When in 1766 the Swedish naturalist Carl Linnaeus updated his Systema Naturae for the twelfth edition, he placed the northern harrier with the falcons and eagles in the genus Falco. Linnaeus included a brief description, coined the binomial name Falco hudsonius and cited Edwards' work. The northern harrier is now placed in the genus Circus, introduced by the French naturalist Bernard Germain de Lacépède in 1799. The genus name Circus is derived from the Ancient Greek kirkos, referring to a bird of prey named for its circling flight (kirkos, "circle"). The specific epithet hudsonius is from "Hudson Bay", the type locality. The species is monotypic: no subspecies are recognised.

The northern harrier was formerly considered to be conspecific with the hen harrier.

The northern harrier is 41–52 cm (16–20 in) long with a 97–122 cm (38–48 in) wingspan. It resembles other harriers in having distinct male and female plumages. The sexes also differ in weight, with males weighing 290 to 400 g (10 to 14 oz), with an average of 350 g (12 oz), and females weighing 390 to 750 g (14 to 26 oz), with an average of 530 g (19 oz). Among standard measurements, the wing chord is 32.8 to 40.6 cm (12.9 to 16.0 in), the tail is 19.3 to 25.8 cm (7.6 to 10.2 in) and the tarsus is 7.1 to 8.9 cm (2.8 to 3.5 in). It is relatively long-winged and long-tailed, having the longest wing and tail relative to its body size of any raptor occurring in North America.

The northern harrier breeds in North America, and its closest relative is the cinereous harrier (C. cinereus) of South America. The male's plumage is darker grey than that of the hen harrier, and the female is also darker and more rufous. The adult male is sometimes nicknamed the "Grey Ghost", because of his striking plumage and spectral aura.

The northern harrier is mostly silent, although males and females will both give fast kek notes that will last 1-2 seconds. When faced with predators or surrounded by smaller birds, they still emit kek notes but at a higher pitch. During the mating season, the female emits a loud scream, which causes the male to either mate or provide food.

This medium-sized raptor breeds on moorland, bogs, prairies, farmland coastal prairies, marshes, grasslands, swamps and other assorted open areas. A male will maintain a territory averaging 2.6 km² (1.0 sq mi), though male territories have ranged from 1.7 to 150 km² (0.66 to 57.92 sq mi).