GŁnter Guillaume (February 1, 1927 - April 10, 1995) a citizen of the German Democratic Republic, was an intelligence agent of that country's secret service, the Stasi. Guillaume emigrated with his wife to West Germany in the 1950s with orders to penetrate the West German political system and report on it. He rose through the hierarchy of the Social Democratic Party of Germany, eventually becoming a close aide to West German chancellor Willy Brandt.

In 1974, Guillaume's spying for the Communist East German government was uncovered by the West German authorities. The resulting scandal led to Brandt having to resign the chancellorship. Guillaume received a thirteen-year prison term for espionage, and his wife an eight-year term. Guillaume was released to the GDR in 1981 in return for Western intelligence agents caught by the Eastern Bloc.

Back in the GDR, Guillaume was celebrated as a hero, worked in the training of spies, and published his autobiography Die Aussage in 1988. Guillaume as well as East German spymaster Markus Wolf have said that the resignation of Brandt was never intended, and the affair counts among the biggest mistakes of the Stasi.