The Mormaer of Caithness was a vassal title mostly held by members of the Norwegian nobility based in Orkney from the Viking Age until 1350. The mormaerdom was held as fief of Scotland and the title was frequently held by the Norse Earls of Orkney, who were thus a vassal of both the King of Norway and the King of Scots. There is no other example in the history of either Norway or of Scotland in which a dynasty of earls owed their allegiance to two different kings.

Earl of Orkney, historically Jarl of Orkney, is a title of nobility encompassing the archipelagoes of Orkney and Shetland, which comprise the Northern Isles of Scotland. Originally founded by Norse invaders, the status of the rulers of the Northern Isles as Norwegian vassals was formalised in 1195. Although the Old Norse term jarl is etymologically related to "earl", and the jarls were succeeded by earls in the late 15th century, a Norwegian jarl is not the same thing. In the Norse context the distinction between jarls and kings did not become significant until the late 11th century and the early jarls would therefore have had considerable independence of action until that time. The position of Jarl of Orkney was eventually the most senior rank in medieval Norway except for the king himself.