Biadoc, also called Biadok and Bjaðök (fl. 1142), was the royal mistress of king Harold IV of Norway and the mother of king Eystein II of Norway.
According to the 19th century history The Clan Donald, the Scottish clan leader Gilledomnan mac Solam had a daughter who was the "wife of Harold, King of Norway".  David Sellar traced this claim to the 18th century and concluded that the Harold concerned may have been Harold IV who was "born and raised in Ireland or the Sudreys [who] claimed to be the son of Magnus Barelegs by a Gaelic mother".
Biadoc's brother was Gillebride of Clan MacInnes, the father of Somerled, Lord of Argyll. On his death bed in 1142, Harold IV declared that he had left a son behind him in Scotland. When Biadoc arrived in Norway the same year and declared her son Eystein to be the son and heir of Harold IV, she was believed; Harold IV himself was identified to be the son of his father the same way.
Harald Gilli's sons' Saga calls her "Biadok". This may be the equivalent of the Gaelic Bethóc, also the name of Somerled's daughter Bethóc, Prioress of Iona.
- ^ Marsden (2008) p. 33 quoting MacDonald A. and MacDonald A. (1896-1904) The Clan Donald. Inverness.
- ^ a b Marsden (2008) p. 33
- ^ Gregory (1881) pp. 9–17
- Gadd, Pia (2009) Frillor, fruar och herrar - en okänd kvinnohistoria (Mistresses, wives and masters - an unknown history of women). (Swedish). Falun.
- Gregory, Donald (1881) The History of the Western Highlands and Isles of Scotland 1493–1625. Edinburgh. Birlinn. 2008 reprint – originally published by Thomas D. Morrison. ISBN 1-904607-57-8
- Marsden, John (2008) Somerled and the Emergence of Gaelic Scotland. Edinburgh. Birlinn. ISBN 978-1-904607-80-9