The title Duke of Portsmouth[1] was a life peerage created (in the Peerage of England) in 1673 for Louise de Kérouaille, one of the mistresses of Charles II. The title, named after Portsmouth, became extinct upon her death in 1734. Her son (by the king) became Charles Lennox, 1st Duke of Richmond in 1675.
Other titles: Countess of Fareham and Baroness Petersfield (for life, 1673)