William Fitzwilliam, 2nd Earl of Southampton (c. 1580–1665), was an English nobleman and politician. He was born into a prominent family and succeeded his father as the Earl of Southampton in 1624. Fitzwilliam served in various political roles, including as a Member of Parliament for Hampshire and later for Winchester. He was a supporter of the Royalist cause during the English Civil War, aligning himself with King Charles I. After the defeat of the Royalists, Fitzwilliam faced challenges, including imprisonment. Despite these difficulties, he remained an influential figure in the political landscape of 17th-century England.