Sir William Portman, 6th Baronet (5 September 1643 – 18 March 1690) FRS was an English politician who sat in the House of Commons between 1661 and 1690. Portman was the son of Sir William Portman, 5th Baronet and his wife Anne Colles, daughter of John Colles. He inherited the baronetcy on the death of his father in 1646. In 1661, Portman was elected Member of Parliament for Taunton for the Cavalier Parliament and held the seat until 1679. In 1679 he was elected MP for Somerset where he sat until 1685. He was then elected MP for Taunton again, and held the seat until his death in 1690.