The Cambridge Union Society, also known as the Cambridge Union, is a historic debating and free speech society in Cambridge, England, and the largest society in the University of Cambridge. The society was founded in 1815 making it the oldest continuously running debating society in the world. Additionally, the Cambridge Union has served as a model for the foundation of similar societies at several other prominent universities, including the Oxford Union and the Yale Political Union. The Union is a private society with membership open to all students of Cambridge University and Anglia Ruskin University. The Cambridge Union is a registered charity and is completely separate from the Cambridge University Students' Union.

The Cambridge Union has a long and extensive tradition of hosting prominent figures from all areas of public life in its chamber, both state- and international-based, including the Dalai Lama, US presidents Theodore Roosevelt and Ronald Reagan, Bill Gates, Stephen Hawking, UK Prime Ministers Winston Churchill, Margaret Thatcher, and John Major, presidential candidate Bernie Sanders, as well as comedian Stephen Fry. Previous presidents of the Cambridge Union have included economist John Maynard Keynes, novelist Robert Harris, politician Kenneth Clarke, and author Arianna Huffington.

The society's origins lie in a dispute among the members of three pre-existing societies, one being a discussion group of which Lord Palmerston was a member. The inaugural meeting of the Cambridge Union was held on 13 February 1815, eight years before the Oxford Union was founded in 1823. However, in the political climate of the Hundred Days which ended the Napoleonic Wars, controversial debate topics generated suspicion among university officials who sought to suppress potentially radical thought among students. On 24 March 1817, university proctors burst into a meeting of the society and the Cambridge Union was temporarily shut down, forbidding future debates from taking place.

By 1821, the Union was permitted to resume debates under strict conditions, including that no political topics relevant to the last twenty years were to be discussed. In 1830, the Union regained its full freedom to debate all topics, excluding those of a strictly theological nature.

The Cambridge Union's Bridge Street premises (52°12′31″N 0°07′10″E / 52.20861°N 0.11944°E / 52.20861; 0.11944) were designed by Alfred Waterhouse (who went on to design the Oxford Union Society's building) and formally opened on 30 October 1866. An additional wing was added several decades later. The future radical Liberal politician, Sir Charles Dilke, was the President chiefly responsible for construction. Included among the building's many rooms are the debating chamber, a dining room, bar, snooker room, the Keynes Library and various offices.

Cambridge escaped virtually undamaged from the widespread aerial bombardment during World War II, ostensibly due to a quid pro quo arrangement with Heidelberg, another historic university city. However, the Union's building was hit during one attack in July 1942. The Union was the only building connected to the university to be directly hit during the war and the explosion caused extensive damage to the Union's library.

In March 1944, XXX Corps commandeered the entire Union building for a week. It is rumoured that during this time the Union became one of a handful of buildings in Cambridge used to plan Operation Overlord and make preparations for the D-Day landings.

Facing financial trouble and extensive rebuilding work, the Union launched a restoration appeal to its life members in October 1945, successfully raising over £3,000 by the end of the academic year.