Male and female white-tailed ptarmigan share similar body size, shape, and winter plumage, with adult body lengths up to 13.4 inches (34 centimeters) and body masses up to 0.9 pound (425 grams). Both the southern and Mt. Rainier white-tailed ptarmigan are white in winter and brown in summer, the feathers changing color with the seasons to camouflage the birds. Although the body feathers change color, the white-tailed ptarmigan is named for its perpetually white tail feathers, a morphological trait that distinguishes it from other ptarmigan species. During the winter, males and females are stark white and difficult to distinguish from the background of snow. In the summer during the breeding season, males turn a lighter color of brown or gray than females and develop a dark band of feathers on the breast that resembles a necklace.

During the winter, the southern and Mt. Rainier white-tailed ptarmigan feed on exposed willows above the snow and dig roosts, or burrows, into the soft snow as protection from wind and weather. During the summer, the white-tailed ptarmigan feeds on willows and other alpine vegetation. White-tailed ptarmigan require cool temperatures, rocky areas, soft snow, and the presence of willows. The southern and Mt. Rainier white-tailed ptarmigan spend their entire lifecycles in alpine habitats and are well adapted to survive in the cold, arid, and open alpine environments. Adaptations of the white-tailed ptarmigan to these alpine environments include its cryptic plumage, low metabolic rates, and heavily feathered feet that act as snowshoes.