Montagu's harrier (Circus pygargus) is a species of harrier, a bird of prey in the family Accipitridae. It breeds in large open areas in the Palearctic and is migratory, wintering in Africa, the Indian subcontinent and parts of the Arabian Peninsula. Its common name commemorates the British naturalist George Montagu.

The first formal description of Montagu's harrier was by the Swedish naturalist Carl Linnaeus in 1758, in the tenth edition of his Systema Naturae, under the binomial name Falco pygargus. The genus Circus was introduced by the French naturalist Bernard Germain de Lacépède in 1799. The genus name is derived from Ancient Greek. Circus is from kirkos, meaning "circle", referring to a bird of prey named for its circling flight (probably the hen harrier), and pygargus is Modern Latin derived from the Greek pugargos, from puge, meaning "rump", and argos, meaning "shining white". The species name was formerly used for the hen harrier before Montagu's was identified as a different species.

Sexual dimorphism is particularly apparent in the plumage of this species. Adult males are characterized by their overall pale grey plumage contrasting with black wingtips. Compared with other harriers this species has characteristic black bands along the secondaries, both above and below the wing and rusty streaks on belly and flanks. Adult females have a broadly similar plumage to that of pallid and hen harriers. The underparts are mostly pale yellow-brown, the belly with longitudinal stripes and the wing coverts spotted. The upper parts are uniform dark brown except for the white upper tail coverts ("rump"), and the sightly paler central wing coverts.

The juvenile plumage resembles that of the female, but differs by the belly and under wing coverts which are not spotted, but uniformly red brown in colour.

A melanistic form occurs regularly in this species. In this form the male is much darker than usual, with a black head, brownish black above and grey underparts. The melanistic female is entirely chocolate brown except for grey flight feathers. Partially melanistic morphs can also be found.

Montagu's harrier has a particularly graceful flight, with powerful and elegant wingbeats which give an impression of buoyancy and ease. In true harrier fashion it searches the countryside, flying low, and generally holds its wings with a marked positive dihedral.

Montagu's harrier is a deceptively small raptor, though it appears larger because of its large wing surface compared to small body weight, which gives it a typically buoyant flight. The female is larger than the male because the female needs to produce eggs, however this is not apparent in the field.

Montagu's harrier can be confused with several species that exist within the same range. The most similar are the hen harrier and the pallid harrier. The male is easily distinguished from other species as its plumage is distinctly darker and more mottled than in the males of hen or pallid harriers. However, distinguishing females and juveniles is more difficult. Usually, Montagu's harrier appears more slender in flight than the hen harrier, with a longer tail, longer and narrower wings and more pointed "hands". Also its flight is more elegant than the hen harrier, with more elastic, almost tern-like wingbeats. The distinction between female pallid and Montagu's harriers is the most delicate and can only be made in good conditions as the proportions are similar. The best recognition character is the pale collar around the neck of female and juvenile pallid harriers which is not present in Montagu's.