John Sadler (of Warmwell) (1615–1674) was an English lawyer, academic, Member of Parliament, Town Clerk of London, Hebraist, Neoplatonist and millennarian thinker, private secretary to Oliver Cromwell, and member of the Parliamentarian Council of State. He was Master of Magdalene College, Cambridge from 1650 to 1660.
Sadler was educated at Emmanuel College, Cambridge.
He married Jane, daughter of the Dorset MP John Trenchard. His sister Ann married John Harvard.