The North Ronaldsay or Orkney is a breed of sheep 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 Auskerry. The Rare Breeds Survival Trust lists the breed as a priority on its 2021–2022 watchlist, and they are in danger of extinction, 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 foreshore by a 1.8 m (6 ft) early 19th-century drystane dyke, which completely encircles the island, forcing the evolution of 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 they subsisted largely on seaweed.

This diet has caused a variety of adaptations in the sheep's digestive system. These sheep now extract the trace element copper far more efficiently than other breeds as their diet of seaweed has a limited supply of copper. This results in them being susceptible to copper toxicity if fed on a grass diet, as copper is toxic to sheep in high quantities. Grazing habits have also changed to suit the sheep's environment. They graze at low tide when the seaweed is exposed and then ruminate at high tide.

A range of fleece colours is exhibited, including grey, brown and red. Meat from the North Ronaldsay has a distinctive flavour, described as "intense" and "gamey", possibly in part due to the high iodine content in their diet of seaweed.

The sheep are descended from the Northern European short-tailed sheep. Their arrival onto North Ronaldsay is not known precisely but it may have been as early as the Iron Age, or possibly even earlier, which would make them potentially the earliest ovines to arrive in Britain. Because of their isolated location, they have evolved without much admixture from imported Roman and European breeds. They share some characteristics, including their colour range and short tails, with Scandinavian sheep introduced when the islands were under Norse control, between the 9th and 15th centuries.

In 1832, a drystane dyke (wall) was erected on the island. Its construction was part of the response to the collapse of the kelping industry, which was the production of soda ash by the burning of seaweed. To provide a livelihood for those previously employed in kelping, the inland farmlands were reorganized, and the sheep kept away from the fields or crofts. Since then, the flocks on the island have been feral. The wall also unintentionally reduced the chances of crossbreeding, which would have diluted the gene pool of an already vulnerable breed. The wall circles the entire coast of the island, 19 km (12 mi), and is 1.8 metres (6 ft) high, making it one of the largest dry stone walls in the world. In 1999, Historic Scotland described it as a "unique and important structure" and designated it an 'A'-list site requiring conservation. This status affords it special protection; any development has to be approved with conservation in mind.

Since the wall was erected, the human population of North Ronaldsay has fallen from 500 to around 50, and current residents lack the manpower to maintain the wall. Successive storms, the most damaging of which was in December 2012, have created large holes in the structure. The cost of repairs has been estimated at several million pounds. In 1902, it cost only 4 pence per hour to repair the wall, using stone taken from the shore. In 2015, Orkney Islands Council reported that some 4.7 km (3 mi) of the wall was in need of work, and that the rate of damage exceeded that of repair.

The punds, or pounds, also listed with Historic Scotland, are nine small enclosures situated across the island to contain the sheep for shearing, counting, lambing and slaughtering. The sheep are herded inside these punds twice a year, the only time they have access to grass feed. Between February and August, the sheep are brought into the punds, once for lambing and once for shearing. The lambs are born on the grass between February and May. At this time, the sheep are counted, lambs are given ear tags, and records are entered with the island's sheep court to record ownership. Shearing takes place in July and August, and the whole island community is involved in herding and shearing the sheep. Slaughtering takes place only in winter when the meat is needed, and when the animals are fatter and yield more meat, since seaweed is more abundant in winter.