The willow ptarmigan is a bird in the grouse subfamily Tetraoninae of the pheasant family Phasianidae. It is also known as the willow grouse and in Ireland and Britain, where the subspecies L. l. scotica was previously considered to be a separate species, as the red grouse. It 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, but 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.

Grouse are a group of birds from the order Galliformes, in the family Phasianidae. Grouse are presently assigned to the tribe Tetraonini, a classification supported by mitochondrial DNA sequence studies, and applied by the American Ornithologists' Union, ITIS, International Ornithological Congress, and others.