The Soay sheep is a breed of domestic sheep (Ovis aries) descended from a population of feral sheep on the 100-hectare (250-acre) island of Soay in the St Kilda Archipelago, about 65 kilometres (40 mi) from the Western Isles of Scotland. It is one of the Northern European short-tailed sheep breeds.

It remains physically similar to the wild ancestors of domestic sheep, the Mediterranean mouflon and the horned urial sheep of Central Asia. It is much smaller than modern domesticated sheep but hardier, and is extraordinarily agile, tending to take refuge amongst the cliffs when frightened. Soays may be solid black or brown, or more often blonde or dark brown with buffish-white underbelly and rump (known as lachdann in Scottish Gaelic, which is cognate to the Manx loaghtan); a few have white markings on the face.

​

In the early twentieth century, some Soay sheep were relocated to establish exotic flocks, such as the flock of "Park Soay" at Woburn Abbey, established by the Duke of Bedford in 1910, and selected for "primitive" characteristics. A number of Soay sheep were translocated from Soay to another of the St Kilda group, the island of Hirta by the Marquess of Bute in the 1930s, after the human population and their sheep were evacuated. The name of the island is from Old Norse Seyðoy, meaning "Island of Sheep". The breed was introduced to and live wild on Holy Isle off Arran.

Soay sheep were introduced from St. Kilda to Lundy, an island in the Bristol Channel, by Martin Coles Harman soon after he purchased the island in 1925. There is also a small population living wild in and around the Cheddar Gorge in Somerset.

The Soays are particularly hardy and have been allowed to become largely feral. The breed is listed in "Category 4: At Risk" by the Rare Breeds Survival Trust, because there are only between 900 and 1500 registered breeding Soay ewes. The Soay is distinct from two other short-tailed breeds also associated with St. Kilda: the Boreray (from Boreray, another of the islands, and formerly also living on Hirta), and the "St. Kilda", a former name for the Hebridean sheep (which is probably not from St. Kilda at all).[