William Greenhill (1591–1671) was an English nonconformist clergyman, independent minister, and member of the Westminster Assembly.
He was born probably in Oxfordshire. At the age of thirteen he matriculated at the University of Oxford on 8 June 1604 and was elected a demy of Magdalen College, Oxford, on 8 January 1605. He graduated B.A. on 25 January 1609, and M.A. on 9 July 1612, in which year he resigned his demyship.
From 1615 to 1633 William Greenhill held the Magdalen College living of New Shoreham, Sussex. He appears to have officiated in some ministerial capacity in the diocese of Norwich, when Matthew Wren was bishop; he got into trouble for refusing to read The Book of Sports. He then moved to London, and was chosen afternoon preacher to the congregation at Stepney, while Jeremiah Burroughes ministered in the morning.
He was a member of the Westminster Assembly of Divines, convened in 1643, and was one of the Independents. In the same year, on 26 April, he preached before the House of Commons of England on occasion of a public fast, and his sermon was published by command of the house, with the title The Axe at the Root. In 1644 he was present at the formation of Stepney Meeting House, the congregational church in Stepney, and was appointed first pastor. In 1649, after the death of Charles, he was appointed by the parliament chaplain to three of the late king Charles's children: James, Duke of York (afterwards James II); Henry, Duke of Gloucester; and the Lady Henrietta Anne.