Sir William Courtenay (1477 – November 1535) "The Great", of Powderham in Devon, was a leading member of the Devon gentry and a courtier of King Henry VIII having been from September 1512 one of the king's Esquires of the Body. He served as Sheriff of Devon three times: from February to November 1522, 1525/6, and 1533/4. He was elected Knight of the Shire for Devon in 1529.
He was the eldest son and heir of Sir William Courtenay (1451–1512) of Powderham by his wife Cecily Cheyne, daughter of Sir John Cheyne of Pinhoe. The family of Courtenay "of Powderham", always known thus until 1556 to distinguish it from the senior line of Courtenay of Tiverton Castle, Earls of Devon, was one of the most influential and best connected in Devon from the 15th century onwards. The cadet line of "Courtenay of Powderham" was descended from Sir Philip Courtenay (1340-1406), a younger son of Hugh Courtenay, 2nd Earl of Devon (1303-1377), of Tiverton Castle, but eventually itself in 1831 was officially recognised by the House of Lords as having become in 1556 de jure holder of the Earldom of Devon inherited after the decease of a distant cousin, Edward Courtenay, 1st Earl of Devon (1527–1556), the last male of the senior line of Courtenay of Tiverton.
William entered his inheritance on 24 November 1512, receiving possession of the family's principal seat, the manor of Powderham, on 11 September. He had lived as a younger man, following his first marriage, in East Coker, Somerset. As a boy he was bound over with another minor ward, Richard Cornwall, to his own recognizance to remain within two miles of the walls of the City of London. The bond was cancelled by April 1512. In 1513 he was appointed Keeper of the royal forest of Petherton Park in Somerset, being summoned to attend upon the King at Easter.