The mountain hare (Lepus timidus), also known as blue hare, tundra hare, variable hare, white hare, snow hare, alpine hare, and Irish hare, is a species of Palearctic hare that is largely adapted to polar and mountainous habitats.

The mountain hare arose during the Late Pleistocene; there is evidence that its range expanded during glaciations into southern Europe, with populations of Iberian hare (Lepus granatensis), European hare (L. europaeus) and broom hare (L. castroviejoi) in northern Iberia harboring mitochondrial haplotypes from the mountain hare. During the Late Pleistocene to Early Holocene, populations of mountain hare in Russia grew at least 10% larger than any living population today. This population has been classified as a distinct species Lepus tanaiticus, but is now generally considered a prehistoric morphotype of the living mountain hare.

This species is distributed from Fennoscandia to eastern Siberia; in addition, isolated mountain populations occur in the Alps, Scotland, the Baltics, northeastern Poland, and Hokkaidō. In Ireland, the Irish hare (L. t. hibernicus) lives on lowland pastures, coastal grasslands, and salt marshes, not just in the mountains. The mountain hare has also been introduced to Iceland, Shetland, Orkney, the Isle of Man, the Peak District, Svalbard, the Kerguelen Islands, the Crozet Islands, and the Faroe Islands. In the Alps, the mountain hare lives at elevations from 700 to 3,800 m (2,300 to 12,500 ft), depending on geographic region and season.

The mountain hare is a large species, though it is slightly smaller than the European hare. It grows to a length of 45–65 cm (18–26 in), with a tail of 4–8 cm (1+1⁄2–3 in), and a mass of 2–5.3 kg (4+1⁄2–11+3⁄4 lb), females being slightly heavier than males. They can live for up to 12 years. In summer, for all populations of mountain hares, the coat is various shades of brown. In preparation for winter most populations moult into a white (or largely white) pelage. The tail remains completely white all year round, distinguishing the mountain hare from the European hare (Lepus europaeus), which has a black upper side to the tail. The subspecies Lepus timidus hibernicus (the Irish mountain hare) is smaller in size and stays brown all year. The Irish hare may also have a "golden" variation, particularly those found on Rathlin Island.

In the Faroe Islands, mountain hares turn grey in the winter instead of white. The winter-grey colour may be caused by downregulation of the agouti hair cycle isoform in the autumn moult.

The diet of the mountain hare varies from region to region. It seems to be somewhat dependent on the particular habitat in which the population under study lives. For example, in northern Scandinavia, where snow may blanket the ground for many months, the hares may feed on twigs and bark. In areas where snowfall is rare, such as Ireland, grass may form the bulk of the diet. Given a choice, mountain hares in Scotland and Ireland seem to prefer grazing (feeding on grasses). For mountain hares on a coastal grassland environment in Ireland, grasses constituted over 90% of their diet. This was higher than the percentage of grass in the diet of the European rabbit (Oryctolagus cuniculus) that inhabited the same environment. The mountain hare is regionally the favourite prey of the golden eagle and may additionally be preyed on by Eurasian eagle-owls and red foxes. Stoats may prey on young hares.

In southern parts of Finland, Norway, and Sweden, the mountain hare and the European hare compete for habitat. The European hare, being larger, is usually able to drive away the mountain hare, but is less adapted for living in snowy regions; its feet are smaller and its winter fur is a mixture of white and brown. While this winter fur is actually a very good camouflage in the coastal regions of Finland where the snow covers the shrubs only a short time, the mountain hare is better adapted for the snowier conditions of the inland areas. The two may occasionally cross.

The Arctic hare (Lepus arcticus) was once considered a subspecies of the mountain hare, but it is now regarded as a separate species. Similarly, some scientists believe that the Irish hare should be regarded as a separate species. Fifteen subspecies are currently recognised.