William St Clair, 8th Baron of Roslin

Sir William St Clair (died circa 1379) was a Scottish noble and by tradition the 8th Baron of Roslin.

Early life Sir William St Clair was the son of Sir William St Clair and the grandson of Sir Henry St Clair who was traditionally the 7th Baron of Roslin, and wife Alice de Fenton. His father was one of the knights chosen to join James Douglas, Lord of Douglas in his expedition to Palestine with the heart of Robert the Bruce where in an encounter with the Saracens, in the Emirate of Granada, Douglas was killed, along with St Clair. The younger William St Clair therefore succeeded his grandfather. 19th century historian Roland Saint-Clair quotes several charters as evidence to show that William St Clair returned safely from the Continent.

Family William St Clair married Isabella (Isobel) of Strathearn, second daughter and eventually the sole heiress of Malise V, Earl of Strathearn, Earl of Orkney and Earl of Caithness, and second wife Marjory of Ross. They had a son, Henry I Sinclair, Earl of Orkney, who was in minority when his father died.