William FitzWilliam or Fitzwilliam may refer to:
William Fitzwilliam (Sheriff of London) (c. 1460–1534), protégé of Cardinal Wolsey William FitzWilliam, 1st Earl of Southampton (c. 1490–1542), English courtier William Fitzwilliam (Dean of Wells) William Fitzwilliam (died 1559) (1506–1559), MP William FitzWilliam (Lord Deputy) (1526–1599), Lord Deputy of Ireland William FitzWilliam, 2nd Baron FitzWilliam (1609–1658), English politician William FitzWilliam, 3rd Viscount FitzWilliam (c. 1610–1670), Irish nobleman William Fitzwilliam, 3rd Earl Fitzwilliam (1719–1756), British peer William Fitzwilliam, 4th Earl Fitzwilliam (1748–1833), British Whig statesman William Wentworth-Fitzwilliam, 6th Earl Fitzwilliam (1815–1902), British peer and Liberal politician William Wentworth-Fitzwilliam, 7th Earl Fitzwilliam (1872–1943), British Army officer, politician and aristocrat William Fitzwilliam, 1st Earl Fitzwilliam (1643–1719), English nobleman, politician, and peer William Fitzwilliam, Viscount Milton (d. 1835), British Member of Parliament for Malton
See also Earl Fitzwilliam, many who were named William FitzWilliam