William Sinclair, 10th Earl of Caithness

William Sinclair, 10th Earl of Caithness (died 1779), was a Scottish nobleman, Earl of Caithness and chief of the Clan Sinclair, a Highland Scottish clan in Caithness.

[1] In 1761, Alexander Sinclair, 9th Earl of Caithness had executed an entail that on the failure of his heirs, his estates should pass to the Sinclairs of Stevenson who were not related to the Sinclairs of Murkle who the Earl was descended from.

Upon his death the male issue of his father, John Sinclair, 8th Earl of Caithness, of his grandfather, Sir James Sinclair, 2nd of Murkle and of his great-grandfather, James Sinclair, 1st of Murkle, became extinct.

However, his grandson, James, who did claim the title, failed to establish legitimacy of his father, David, the son of David Sinclair of Broynach.

William Sinclair of Rattar was therefore served male heir, becoming 10th Earl of Caithness, with the Committee of Privileges adjudging the title to him in May 1772.