Algernon Sidney, British philosopher (d. 1683)
Algernon Sidney or Sydney (15 January 1623 – 7 December 1683) was an English politician, republican political theorist and colonel. A member of the middle part of the Long Parliament and commissioner of the trial of King Charles I of England, he opposed the king's execution. Sidney was later charged with plotting against Charles II, in part based on his most famous work, Discourses Concerning Government, which was used by the prosecution as a witness at his trial. He was executed for treason. After his death, Sidney was revered as a "Whig patriot—hero and martyr".

The works of Algernon Sidney, along with those of contemporary John Locke, are considered a cornerstone of western thought. Discourses Concerning Government cost Sidney his life. However, the ideas it put forth survived and ultimately culminated in the Glorious Revolution in England and the founding of the United States. Sidney directly opposed the theory of divine right of kings by suggesting ideas such as limited government, voluntary consent of the people and the right of citizens to alter or abolish a corrupt government. Discourses Concerning Government has been called "the textbook of the American revolution."