Thomas Howard, 5th Duke of Norfolk (9 March 1627 – 13 December 1677) was an English nobleman who from 1645 was deemed a lunatic. Born the eldest son of Henry Howard, 15th Earl of Arundel, Howard left England to study at Utrecht University at the start of the English Civil War. While visiting his paternal grandfather at Padua in 1645 he contracted a fever that damaged his brain. He was declared insane and confined in Padua with a physician caring for his needs. He became Earl of Arundel upon the death of his father in 1652.

Unable to coherently manage his English estates, the running of them was given over to his next eldest brother, Henry Howard, who acted in his place. In 1660 Henry successfully petitioned the House of Lords to have the attaindered title Duke of Norfolk restored. Howard, as eldest son in a line descended from Thomas Howard, 4th Duke of Norfolk, became 5th Duke of Norfolk. He never returned to England, being kept at Padua until his death in 1677.

Howard's younger brothers and uncle, William Howard, 1st Viscount Stafford, questioned his lunacy several times. It was suggested that Henry was holding Howard at Padua in bad faith in order to reap the benefits of representing him in England. Parliament unsuccessfully ordered Howard to return in 1659, and two petitions from his brothers in 1675 and 1677 to do the same were turned down. He died childless; his family ensured that he never married so that he could not produce an heir who might inherit his mental disorder.

Howard died at Padua on 13 December 1677, ending worries that he would surprisingly outlive his brother.[27][13] Henry, who had been created Baron Howard of Castle Rising in 1669 and Earl of Norwich and Earl Marshal in 1672, succeeded him as 6th Duke of Norfolk.[27] Howard's body was brought back to England a year after his death, being buried at Arundel on 11 December 1678.[17][1