Lord of the Isles or King of the Isles (Scottish Gaelic: Triath nan Eilean or Rìgh Innse Gall) is a title of Scottish nobility with historical roots that go back beyond the Kingdom of Scotland. It began with Somerled in the 12th century and thereafter the title was held by a series of his descendants, the Norse-Gaelic rulers of the Isle of Man and Argyll and the islands of Scotland in the Middle Ages. They wielded sea-power with fleets of galleys (birlinns). Although they were, at times, nominal vassals of the kings of Norway, Ireland, or Scotland, the island chiefs remained functionally independent for many centuries. Their territory included much of Argyll, the Isles of Arran, Bute, Islay, the Isle of Man, Hebrides, Knoydart, Ardnamurchan, and the Kintyre peninsula. At their height they were the greatest landowners and most powerful lords after the kings of England and Scotland.

The Norse–Gaels were a people of mixed Gaelic and Norse ancestry and culture. They emerged in the Viking Age, when Vikings who settled in Ireland and in Scotland became Gaelicised and intermarried with Gaels. The Norse–Gaels dominated much of the Irish Sea and Scottish Sea regions from the 9th to 12th centuries. They founded the Kingdom of the Isles, the Kingdom of Dublin, the Lordship of Galloway, and briefly ruled the Kingdom of York. The most powerful Norse–Gaelic dynasty were the Uí Ímair or House of Ivar.