Sir William FitzWilliam (1506 – 3 October 1559), of Windsor, Berkshire, was an Irish courtier and Member of Parliament in England.
His mother, Eleanor, was the daughter of Sir John Dowdall, of Newtown, County Kildare, and co-heir to her mother, Margaret, daughter of Sir Jenico d'Artois of Ardglass Castle and Strangford Castle, County Down.
[1] His mother was the step-daughter of Rowland FitzEustace, 1st Baron Portlester, father-in-law of Gearóid Mór FitzGerald, 8th Earl of Kildare.
His brother entered into the service of their cousin, Gerald FitzGerald, 9th Earl of Kildare, and became Gentleman of the Bedchamber to Henry VIII.
He is buried with a monument in the Bray Chantry at St George's Chapel, Windsor Castle.