English Lutheran who was martyred after being used by King Henry VIII to gain support for his antipapal campaign in England.

Barnes, a prior of the Austin Friars at Cambridge, was early influenced by reformist views and ruined a promising academic career when on Christmas Eve, 1525, he preached a sermon attacking clerical worldliness. Pressure from the university authorities caused him officially to abjure his heretical opinions, but in 1528 he escaped to Wittenberg, in Germany, where he formed an enduring friendship with Martin Luther. Beginning in 1531 Henry VIII's chief minister, Thomas Cromwell (Cromwell, Thomas, Earl of Essex, Baron Cromwell of Okeham), obtained for Barnes safe conduct for frequent diplomatic trips between Germany and England. Henry and Cromwell seem to have had no respect for Barnes; instead, they exploited his close contact with the Lutherans in order to bolster their drive to make the king, rather than the pope, the head of the church in England.

The fall of Cromwell in June 1540 removed Barnes's sole protector; in July he was burned as a heretic, though he had never been accorded a trial. Historians have generally viewed him as a sincere but rash and somewhat unstable man. His most important writings are A Supplication to Henry VIII (1531), Vitae Romanorum Pontificum (1535; “Lives of the Roman Pontiffs”), and Confession of Faith (1540).