Igor Gouzenko (January 13, 1919–1982) was a cipher clerk for the Soviet Embassy to Canada in Ottawa, Ontario. He defected on September 5, 1945 with 109 documents on Soviet espionage activities in the West.

Gouzenko's defection exposed Joseph Stalin's efforts to steal nuclear secrets and the then unknown technique of planting sleeper agents. As a result, with World War II over, the Soviet's position as an ally to the West changed and the Cold War began.

Gouzenko was born in the Soviet Union. At the start of the World War II he joined the military where he trained as a cipher clerk. In 1943 he came to Ottawa where for two years he coded and deciphed incoming and outgoing messages. His position as cipher clerk gave him access to Soviet espionage activities in the West.

In 1945, hearing that he and his family were to be sent home to the Soviet Union and dissatisfied with the quality of life and the politics of his homeland, he decided to defect. He went to the RCMP but his story was not believed. He then went to the Ottawa Journal newspaper, but the paper's night editor wasn't interested, and suggested he go to the justice ministry, where nobody was on duty. Terrified that the Soviets had discovered his duplicity, he went back to his apartment and hid his family in the apartment across the hall for the night. Gouzenko watched through the keyhole as a group of Soviet agents broke into his apartment and began searching through his belongings.

The next day Gouzenko was able to find contacts in the RCMP who could understand his evidence, which led to the arrest of twelve Soviet spies, famously including Alan Nunn May, and to a royal commission on espionage in Canada. Even more importantly it alerted other countries around the world, such as the United States and the United Kingdom, that Soviet agents had almost certainly infiltrated their nations as well.

Gouzenko and his family were given another identity by the Canadian government out of fear of Soviet reprisals. Little is known about his life afterwards, but it is understood that he settled down to a middle class existence somewhere in Canada. Gouzenko managed to keep in the public eye, however, writing two books, This Was My Choice a non-fiction account of his defection, and a novel The Fall of a Titan which won a Governor General's Award in 1954. Gouzenko also appeared occasionally on television, always with a white cloth draped over his head.

Gouzenko died of a heart attack in 1982.