The snow bunting (Plectrophenax nivalis) is a passerine bird in the family Calcariidae. It is an Arctic specialist, with a circumpolar Arctic breeding range throughout the northern hemisphere. There are small isolated populations on a few high mountain tops south of the Arctic region, including the Cairngorms in central Scotland and the Saint Elias Mountains on the southern Alaska-Yukon border, as well as the Cape Breton Highlands. The snow bunting is the most northerly recorded passerine in the world.

The snow bunting was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae. He placed it with the buntings in the genus Emberiza and coined the binomial name Emberiza nivalis. He specified the locality as Lapland. It is now placed in the genus Plectrophenax, described in 1882 by the Norwegian born zoologist Leonhard Stejneger with the snow bunting as the type species. The genus name Plectrophenax is from Ancient Greek plektron, "cock's spur", and phenax "imposter", and the specific nivalis is Latin for "snow-white".

The snow bunting was formerly classified in the family Emberizidae, which included American sparrows, buntings, towhees and finches. All these species came into existence after a broad geologically recent radiation of passerine birds. However, it is now part of the narrower family Calcariidae, which also contains the longspurs. Despite the wide distribution of this species there are only very small differences between different phenotypes.

Four subspecies are accepted, which differ slightly in the plumage pattern of breeding males:

It is very closely related to the Beringian McKay's bunting (Plectrophenax hyperboreus), which differs in having even more white in the plumage. Hybrids between the two occur in Alaska, and they have been considered conspecific by some authors, though they are currently generally treated as separate species.

A hybrid with a Lapland longspur was photographed at St. Lewis Inlet, Newfoundland and Labrador, Canada, during spring migration in April 2011.

The snow bunting is a sexually dimorphic, medium-sized passerine bird. It is ground-dwelling, and walks, runs and can hop if needed. It is fairly large and long-winged for a bunting. It measures 15 cm with a wingspan of 32–38 cm (13–15 in) and weights 30 to 40 grams. The bill is normally yellow with a black tip, however in males, the bill is all black in the summer. The plumage is white underneath and the wings and back have black and white on them. The female and male have different plumage; during the breeding season, the male is white with black wingtips and a black back, while the female has black wingtips and the black feathers on the back fringed with brown. During the winter, they both have broad orangey-brown fringes on the back feathers. In the spring, the buntings do not moult as many other passerines do; instead the breeding colours come with the wearing and abrasion of the brown fringes to show just the black centres of the feathers. Unlike most passerines, it has feathered tarsi, an adaptation to its harsh cold environment. No other passerine can winter as far north as this species, apart from the common raven.

This species is easily confused with McKay's bunting due to the similarity of their plumage and the occurrence of hybrids. It can also be confused with the Lapland longspur, which differs mainly in having minimal white in the wing; their calls are similar, but the snow bunting has more 'liquid' rippling tone, while Lapland longspur has a drier, more rattling tone. Another species that it may occur with is the horned lark, although that has a conspicuous black-and-yellow head pattern, a longer, partly black tail and no white on the wings.