William Wentworth Fitzwilliam, 4th Earl Fitzwilliam in Ireland, 2nd Earl Fitzwilliam in Great Britain (30 May 1748 - 8 February 1833) was a British politician of the late 18th and early 19th centuries.

Fitzwilliam, who inherited his peerage at the age of eight, studied at Eton College, where he became friends with Charles James Fox and Lord Morpeth, who would later become Earl of Carlisle.

Fitzwilliam was the nephew of Lord Rockingham, leader of the Old Whig opposition faction in the 1760s and 1770s, and when Rockingham died in 1782, Fitzwilliam inherited his estates (making him one of the greatest landowners in the country), as well as taking up his uncle's role as a major leader of the Whigs. In the Lords, Fitzwilliam was one of the leading supporters of the Fox-North coalition government, being considerably more effective than the nominal leader of the government, the Duke of Portland. Fitzwilliam was to have become head of the India board under the ministry's ill-fated India bill, but the failure of the bill led to the fall of the ministry, and Fitzwilliam found himself in opposition.

After the fall of the coalition, Fitzwilliam became one of the leading opposition figures in the House of Lords, and generally played the role of a Whig grandee. A fairly conservative Whig, Fitzwilliam was horrified by the excesses of the French Revolution, but also concerned to maintain party unity and his own friendship with Fox. Fitzwilliam nevertheless supported the war against the French, and agreed with the decision of Portland, the leader of the anti-Foxite Whigs, to break with Fox and his supporters and support Pitt. Nevertheless, Fitzwilliam was reluctant for the Portland group to actually join the government, although he ultimately joined on as Lord President in July 1794. Shortly thereafter, he was made also Lord Lieutenant of Ireland. In Ireland, Fitzwilliam attempted to push through Catholic emancipation, and aroused controversy in the cabinet by his attempts to provide patronage to Irish Whigs. He was dismissed almost immediately upon arriving in February 1795.

After his dismissal, Fitzwilliam, blaming Portland for his dismissal, returned to opposition and eventually reconciled with Fox. In the Ministry of All the Talents of 1806 to 1807, Fitzwilliam was once again Lord President, and then minister without portfolio, and he continued as a leading Whig in opposition, although he became gradually less politically involved, and did not join the government when the Whigs finally returned to power in 1830. He died in 1833.

|- style="text-align: center;" | width="30%" |Preceded by:
The Earl of Westmorland | width="40%" style="text-align: center;" |Lord Lieutenant of Ireland
1794–1795 | width="30%" |Succeeded by:
The Earl Camden

|- style="text-align: center;" | width="30%" |Preceded by:
The Viscount Sidmouth | width="40%" style="text-align: center;" |Lord President of the Council
1806 | width="30%" |Succeeded by:
The Viscount Sidmouth

|- style="text-align: center;" | width="30%" |Preceded by:
— | width="40%" style="text-align: center;" |Minister without Portfolio
1806–1807 | width="30%" |Succeeded by:
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