The North Ronaldsay or Orkney is a breed of sheep (Ovis aries) from North Ronaldsay, the northernmost island of Orkney, off the north coast of Scotland. It belongs to the Northern European short-tailed sheep group of breeds, and has evolved without much cross-breeding with modern breeds. It is a smaller sheep than most, with the rams (males) horned and ewes (females) mostly hornless. It was formerly kept primarily for wool, but now the two largest flocks are feral, one on North Ronaldsay and another on the Orkney island of Linga Holm. The Rare Breeds Survival Trust lists the breed as "vulnerable", with fewer than 600 registered breeding females in the United Kingdom.

The semi-feral flock on North Ronaldsay is the original flock that evolved to subsist almost entirely on seaweed – they are one of few mammals to do this. They are confined to the shoreline by a 1.8 m (6 ft) tall drystane dyke, which completely encircles the island, forcing the sheep to evolve this unusual characteristic. The wall was built as kelping (the production of soda ash from seaweed) on the shore became uneconomical. Sheep were confined to the shore to protect the fields and crofts inside, and afterwards subsisted largely on seaweed.

The Orkney sheep's meat has Protected Geographical Status in European Union law, so only meat from North Ronaldsay sheep can be marketed as Orkney Lamb.

North Ronaldsay sheep in "The Sea Mother's Gift"[]

In 1161 BC, Orcadian sailors brought back an unusual batch of sheep from the Great Island, along with a young shepherdess ("Gefalal the stranger") who was the sole survivor of a pirate raid on her village. After the Day of Darkness devastated the islands in 1160 BC, these sheep demonstrated an unusual dietary habit which helped to save Orcadian civilization.