English statesman who played a leading role in the Revolution of 1688–89.

The son of Robert Sidney, 2nd earl of Leicester, he entered Parliament in 1679 and supported legislation to exclude King Charles II's Roman Catholic brother James, duke of York (later King James II), from the succession to the throne. Sent by Charles as envoy to The Hague, Sidney used the opportunity to cultivate the friendship of the Protestant ruler William of Orange (William III) in the hope that William could eventually be accepted as Charles II's successor. Charles triumphed over the exclusionists in 1681, however, and Sidney fell into disfavour. After James II came to power in 1685, Sidney lived on the Continent for nearly three years. He secretly encouraged William to seize the English throne, and in December 1687 he returned to England, at William's request, in order to rally support for William among the nobility. He signed, and may have drafted, the document inviting William to take power in England (June 30, 1688), and he sailed from Holland with William in the expedition that landed at Tor Bay, Devon, and deposed James.

Sidney served the new king—with notable lack of success—as secretary of state and then as lord lieutenant of Ireland (1692–93). Created a baron and viscount in 1689 and earl of Romney in 1694, he continued to hold minor posts until the accession of Queen Anne in 1702. His contemporaries viewed him as a proud and drunken man. He died unmarried, and his peerage became extinct.