Thomas of Lancaster, 1st Duke of Clarence (autumn 1387 – 22 March 1421) was a medieval English prince and soldier, the second son of King Henry IV of England, brother of Henry V, and heir to the throne in the event of his brother's death.
He acted as councillor and aide to both.
After the death of his father, he participated in the military campaigns of his brother in France during the Hundred Years' War.