Sir Thomas Courtenay, 6th Earl of Devon (1432–1461) was the eldest son of Thomas de Courtenay, 5th Earl of Devon by Margaret Beaufort, the royal blooded daughter of John Beaufort, Earl of Somerset and Margaret Holland, daughter of Thomas Holland, Earl of Kent.
Thomas Courtenay was aged 26 years when his father died during the first phase of the Wars of the Roses. On 19 December 1459 he was appointed Keeper of Exmoor Forest, as his father had been before him. The Courtenay family held extensive lands throughout Devon, and were one of the greatest landowners in the south-west, comparable only to the Paulet family; the later Earls of Winchester. Thomas was a staunch adherent of the House of Lancaster, as was the family tradition, until his father's feud with William Bonville. Thomas was a leading exponent of violent conflagration, leading the raid on the house at Upcott, and subsequent murder of the respected lawyer, Nicholas Radford. He took, with his brother, internecine warfare to new levels of brutality. His subsequent adherence to Queen Margaret's affinity was his undoing. Despite all he was appointed as Keeper of Exmoor Forest in 1459. Devon's decision was advised by his dislike of Bonville, who sought Warwick's patronage. He was fighting alongside John Beaufort in the northern army and was defeated at the "bloodiest battle on English soil" at Towton, where he was taken prisoner.
He was beheaded on the orders of Edward IV on 3 April 1461 at York. He was attainted by Parliament, in which document his family forfeited their earldom and barony of Courtenay and their possessions. His younger brother Henry was given the manor of Topsham in Devon on 27 July 1461, in compensation for these forfeitures. Henry himself was executed in 1468.