The willow ptarmigan (/ˈtɑːrmɪɡən/ TAR-mi-gən; Lagopus lagopus) is a bird in the grouse tribe Tetraonini of the pheasant family Phasianidae. It is also known as the willow grouse. The willow ptarmigan breeds in birch and other forests and moorlands in northern Europe, the tundra of Scandinavia, Siberia, Alaska and Canada, in particular in the provinces of Newfoundland and Labrador and Quebec. It is the state bird of Alaska.

In the summer the birds are largely brown, with dappled plumage, while in the winter they are white with some black feathers in their tails. The species has remained little changed from the bird that roamed the tundra during the Pleistocene. Nesting takes place in the spring when clutches of four to ten eggs are laid in a scrape on the ground. The chicks are precocial and soon leave the nest. While they are young, both parents play a part in caring for them. The chicks eat insects and young plant growth while the adults are completely herbivorous, eating leaves, flowers, buds, seeds and berries during the summer and largely subsisting on the buds and twigs of willow and other dwarf shrubs and trees during the winter.

The willow ptarmigan was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the binomial name Tetrao lagopus. He specified the type locality as "Europae alpinis" but this has been restricted to Sweden. The willow ptarmigan is now one of four species placed in the genus Lagopus that was introduced in 1760 by the French zoologist Mathurin Jacques Brisson. The word lagopus is Latin for a ptarmigan. The word originally came from Ancient Greek λαγωπους/lagōpous meaning "hare-footed", from λαγως/lagōs meaning "hare" and πους/pous, ποδος/podos meaning "foot".

The taxonomy is confused, partly because of the complicated changes in plumage several times a year and the differing color and pattern of the summer plumage:

Fifteen subspecies are recognised:

The willow ptarmigan often hybridises with the black grouse (Tetrao tetrix) and the hazel grouse (Tetrastes bonasia) and occasionally with the western capercaillie (Tetrao urogallus), the spruce grouse (Falcipennis canadensis) and the rock ptarmigan (Lagopus muta).

During the Pleistocene, the willow ptarmigan widely occurred in continental Europe. Authors who recognize paleosubspecies have named the Pleistocene willow ptarmigan L. l. noaillensis (though the older name medius might be the correct one). These marginally different birds are said to have gradually changed from the earlier (Pliocene) Lagopus atavus into the present-day species L. lagopus. Pleistocene willow ptarmigan are recorded from diverse sites until the end of the Vistulian glaciation about 10,000 years ago, when the species, by then all but identical with the living birds, retreated northwards with its tundra habitat. Additionally, mitochondrial DNA of the willow ptarmigan has been recovered from permafrost in the Yukon Territory.

The willow ptarmigan is a medium to large ground-dwelling bird and is the most numerous of the three species of ptarmigan. Males and females are about the same size, the adult length varying between 35 and 44 centimetres (14 and 17 in) with a wingspan ranging from 60 and 65 centimetres (24 and 26 in). The weight is 430 to 810 grams (15 to 29 oz). It is deep-chested and has a fairly long neck, a broad bill, short feathered legs and a moderately short rounded tail. In the summer, the male's plumage is marbled brown, with a reddish hue to the neck and breast, a black tail and white wings and underparts. It has a red semicircular comb above each eye, which become red and prominent in the breeding season. The female is similar in appearance but with much smaller eye combs and has brown feathers scattered among the white feathers on her belly. During winter, the body plumage and two central tail feathers of both sexes becomes completely white, except for the black outer rectrices. Their wing feathers remain white all year round. Immature birds resemble the adults.