The Cambridge Union Society, commonly referred to simply as the Cambridge Union, is a debating society in Cambridge, England and is the largest society at the University of Cambridge. Throughout its nearly two centuries of continuous operation, the Union has developed a worldwide reputation as a noted symbol of free speech and open debate. Additionally, as one of the oldest organizations of its kind in the world, the Cambridge Union served as a model for the subsequent foundation of similar societies at several other prominent universities including the Oxford Union and the Yale Political Union.

The Cambridge Union was founded on February 13, 1815 as a union of three debating societies and quickly rose to prominence in University life. Early officers have included the historian and essayist, Thomas Babington Macaulay and many subsequent officers have gone on to become influential leaders in a wide variety of fields and professions. Just a few years after it was founded, the Union was even temporarily shut down by the University for being too contentious. The Union is legally a self-funded private members' club which owns and has full control over its private property and buildings in the Cambridge city centre. However, it enjoys strong relations with the university, allowing other societies to hire rooms out, often admitting guests to its events and even holding a debate open to all students once a year.

After nearly 200 years, the Cambridge Union is still best known for its debates which often receive national or international media attention. However, it also organises lectures by visiting speakers, film evenings and other social events for its members. The top members of its debating team compete internationally against other top debating societies, and Cambridge regularly fields one of the most successful teams at the World Universities Debating Championships.

Its current Bridge Street premises ( ) were designed by Alfred Waterhouse and formally opened on October 30, 1866, with an additional wing to the building added several decades later. The future radical Liberal politician, Sir Charles Dilke, was the President largely responsible for construction. Included among the building's many rooms are the famous debating chamber, a dining room, bar, snooker room, the Keynes Library and various offices. The society offers hot refreshments and newspapers during the day, as well a full bar during the evenings. Recently, the Society's building has proved to be a popular filming location with scenes for several British television programs and an upcoming feature length motion picture shot on the premises.

The Cambridge Union is sometimes confused with the Cambridge University Students' Union, the representative body for students set up much more recently in 1971. Consequently, the term 'President of the Union' often causes confusion, for example, in reference to Charles Clarke, a former president of the students' union .