Thomas de Mowbray, 1st Duke of Norfolk

Thomas de Mowbray, 1st Duke of Norfolk (22 March 1366 – 22 September 1399) was an English nobleman.

Mowbray was the son of John de Mowbray, 4th Baron Mowbray (died 1368), and Elizabeth de Mowbray, Baroness Mowbray and suo jure 5th Baroness Segrave (died 1375). She was the eldest daughter of John de Segrave, 4th Baron Segrave and Margaret Plantagenet, Duchess of Norfolk, who was the eldest daughter of Thomas of Brotherton, 1st Earl of Norfolk, a son of Edward I of England and his second Queen consort Marguerite of France. Thus Mowbray was a great-great-grandson of King Edward I.

On 10 February 1382, he succeeded his brother John as 6th Baron Mowbray and 7th Baron Segrave, and soon afterwards was created Earl of Nottingham, a title that had also been created for his elder brother. Three years later he was appointed Earl Marshal of England, and in that capacity he fought against the Scots and then against the French.

Lord Nottingham was one of the Lords Appellant to King Richard II who deposed some of King Richard's court favorites in 1387. The King's uncle, Thomas of Woodstock, 1st Duke of Gloucester, was imprisoned at Calais, where Nottingham was Captain. When Gloucester was killed in 1397, it was probably at the King's orders and probably with Nottingham's involvement. A few weeks later he was created Duke of Norfolk, though his aged grandmother, the Duchess of Norfolk, was still alive. When she died the next year he also became 3rd Earl of Norfolk.

Later, in 1398, Norfolk quarrelled with Henry of Bolingbroke, 1st Duke of Hereford (later King Henry IV), apparently due to mutual suspicions stemming from their roles in the conspiracy against the Duke of Gloucester. The King banished them both. After Hereford returned and usurped the throne, Norfolk was stripped of the Dukedom of Norfolk, though he retained his other titles. He died of the plague in Venice, on 22 September 1399.

The matter of Norfolk's quarrel and subsequent banishment is depicted at the beginning of Shakespeare's Richard II.

Norfolk had no children by his first wife, Elizabeth le Strange, suo jure 3rd Baroness Strange, daughter and heiress of John le Strange, 2nd Baron Strange. He had two sons by his second wife, Lady Elizabeth FitzAlan, daughter of Richard FitzAlan, 11th Earl of Arundel: Thomas, later 4th Earl of Norfolk; and John, later 5th Earl of Norfolk, later restored as 2nd Duke of Norfolk.