Mary, Princess Royal (Mary Henrietta Stuart; 4 November 1631 – 24 December 1660), was a British princess, a member of the House of Stuart, and by marriage Princess of Orange and Countess of Nassau. She acted as regent for her minor son from 1651 to 1660. She was the first holder of the title Princess Royal.
Mary was the eldest daughter of King Charles I of England and Queen Henrietta Maria, and sister of Charles II and James II. She was married to the future stadtholder of the Netherlands, William II of Orange, in 1641 at the age of nine, while he was fourteen. Initially, she remained in England with her parents because of the heated political situation in England until early 1642, when she and her mother left for the Netherlands. Five years later in 1647, Mary's husband inherited the titles of Prince of Orange and Stadtholder of Holland, Zeeland, Utrecht, Guelders, Overijssel and Groningen in the United Provinces of the Netherlands.