Isabella Bennet FitzRoy, Duchess of Grafton and later 2nd Countess of Arlington suo jure (c. 1668 – 7 February 1723), was a British peer and heiress.

Elisabeth was a daughter of Louis of Nassau-Beverweerd[1] and thus a granddaughter of Maurice of Nassau, Prince of Orange, and a great-granddaughter of William the Silent.

She was married at the age of four to Henry FitzRoy, Earl of Euston (later created Duke of Grafton), the nine-year-old illegitimate son of King Charles II.

[2] Isabella and her husband had one son, Charles FitzRoy, who succeeded his parents as 2nd Duke of Grafton and 3rd Earl of Arlington.

After her husband's death in 1690 from a wound received at the storming of Cork while leading the forces of William of Orange, she remarried on 14 October 1698 to Sir Thomas Hanmer, 4th Baronet, Speaker of the House of Commons.