Sir William Cavendish (1505 - 25 October 1557) was an English courtier who became one of Thomas Cromwell's "visitors of the monasteries" when King Henry VIII annexed the property of the Catholic Church at the end of the 1530s, and Cavendish became quite wealthy from his share of those properties.

His first two wives died, leaving him with two daughters, and in 1547 he married Bess of Hardwick. In the ten years before he died, they had eight children, only six of whom survived infancy. Some of their descendants became the Dukes of Devonshire, and others became Dukes of Newcastle. Their grand-daughter Arabella Stuart was a claimant to the throne of England in 1603.

During the reign of Mary I of England, a favorable biography of Thomas Cardinal Wolsey was first published, written from the perspective of one of his closest aides, the one who had taken King Henry news of Wolsey's death. Although for centuries Sir William was said to be its author, historians now attribute it to his older brother George Cavendish (1500 - ~1562) instead.

Several more persons going by the name William Cavendish are listed in the articles Duke of Devonshire and Duke of Newcastle.