The Gouzenko Affair was the name given to events in Canada surrounding the defection of Igor Gouzenko, a GRU cipher clerk stationed at the Soviet Embassy in Ottawa, from the Soviet Union in 1945 and his allegations regarding the existence of a Soviet spy ring of Canadian communists. Gouzenko's defection and revelations are considered by historians to have marked the beginning of the Cold War in Canada, as well as potentially setting the stage for the "Red Scare" of the 1950s.

Igor Sergeyevich Gouzenko was a cipher clerk for the Soviet embassy to Canada in Ottawa, Ontario, and a lieutenant of the Soviet Main Intelligence Directorate (GRU). He defected on September 5, 1945, three days after the end of World War II, with 109 documents on the USSR's espionage activities in the West. In response, Canada's Prime Minister, Mackenzie King, called a royal commission to investigate espionage in Canada.