Sir William Courtenay, Knight, (1553 – 24 June 1630) of Powderham in Devon was a prominent member of the Devonshire gentry. He was Sheriff of Devon in 1579–80 and received the rare honour of having been three times elected MP for the prestigious county seat (Devon) in 1584, 1589 and 1601.
He was the son and heir of Sir William Courtenay (c. 1529 – 1557) of Powderham, MP for Plympton Erle in 1555, by his wife Elizabeth Paulet, daughter of John Paulet, 2nd Marquess of Winchester. He was 9th in descent from Hugh de Courtenay, 2nd Earl of Devon (1303–1377), of Tiverton Castle, feudal baron of Okehampton and feudal baron of Plympton, one of whose younger sons was Sir Philip Courtenay (1340–1406) of Powderham, founder of the junior line of Courtenay of Powderham.
In 1557 at the age of 4 he succeeded his father. He trained as a lawyer in the Middle Temple.
Sir William was knighted on 25 March 1576, served as Sheriff of Devon for 1579–80 and was also involved in the Munster Plantation in Ireland in the 1580s. Sir William was elected Member of Parliament for Devon in 1584, 1589 and 1601. In 1831 he was recognised by a retrospective decision of the House of Lords as having been de jure 3rd Earl of Devon.