Algernon Sydney (or Sidney), (January 1623 – December 7 1683), was an English politician, an opponent of King Charles II of England.

A son of the Earl of Leicester, and the great-nephew of Sir Philip Sidney, he is thought to have been born at Penshurst Place in Kent. During the English Civil War, he joined the army of Parliament, but became critical of Oliver Cromwell's leadership. Following the Restoration of the monarchy, he went into exile, returning in 1677. In 1683, he was implicated in the Rye House Plot, and was found guilty of treason and executed.

His writings were collected and published posthumously under various titles:

• Discourses
• Discourses on Government
• Discourses Concerning Government
• Discourses Concerning Civil Government