Soay sheep are a primitive breed of domestic sheep found primarily on the remote islands of St. Kilda, west of Scotland. They are known for their small size, hardy nature, and distinctive appearance, which includes a mix of dark brown, black, and light cream-coloured wool. They have a wild, almost goat-like look, with both rams and ewes sporting short, curved horns. Because they haven't been selectively bred as modern sheep have, Soay sheep retain characteristics more similar to their ancient ancestors, including a natural shedding of wool, which means they don't typically need shearing.

Soay sheep have adapted to thrive in harsh environments, grazing on sparse vegetation in rocky and windy terrains. Their hardiness has made them a subject of interest in evolution, genetics, and natural selection studies, especially in isolated populations where they evolve without much human intervention. Due to this, the St. Kilda population is frequently studied to understand their adaptability, survival, and breeding behaviours in a natural environment.