Jan Oskar Sverre Lucien Henri Guillou (Swedish pronunciation: [ˈjɑːn ɡɪˈjuː], French: [ɡiju]; born 17 January 1944) is a French-Swedish author and journalist. Guillou's fame in Sweden was established during his time as an investigative journalist, most notably in 1973 when he and co-reporter Peter Bratt exposed a secret, immoral and illegal intelligence organization in Sweden, Informationsbyrån (IB). He is still active within journalism as a column writer for the Swedish evening tabloid Aftonbladet. Among his books are a series of spy fiction novels about a spy named Carl Hamilton, and a trilogy of historical fiction novels about a Knight Templar, Arn Magnusson. He is the owner of one of the largest publishing companies in Sweden, Piratförlaget (Pirate Publishing), together with his wife, publisher Ann-Marie Skarp, and Liza Marklund.

Guillou was born in Södertälje, Stockholm County, Sweden. His Breton-Swedish father, Charles Guillou (1922–2020), came to Sweden in 1941 as the son of a member of the French Resistance and head of the offices of Free France in Stockholm, and later became a journalist for the French nationwide daily newspaper L'Équipe. His mother, Marianne (née Botolfsen; 1922–2013), was of Norwegian descent. Guillou acquired French citizenship at birth and became a Swedish citizen in 1975. When Guillou's paternal grandfather was offered a position at the French embassy in Helsinki, Finland, his father decided to move with him and settled there. Guillou grew up with his mother and her new husband in Saltsjöbaden and Näsbypark outside of Stockholm.

Guillou studied at Vasa Real in Stockholm but was expelled from the school because of his bad behaviour, including physical abuse, theft and blackmail. He then went on to study for two years at the Solbacka boarding school in Södermanland. Guillou has described his upbringing, with the continuous physical abuse from his sadistic stepfather and the harsh treatment at the Solbacka school, in the semi-autobiographical novel Ondskan (1981). According to the Swedish tabloid newspaper Expressen, his mother, his sister, his teachers and his friends from the Solbacka school have contested his account and called the book a hoax.

He finished his studentexamen (upper-secondary final examination) from the boarding school Viggbyholmsskolan, located in Viggbyholm, in 1964. Guillou then attended Stockholm University from 1964 to 1966.

Guillou first lived together with author and translator Marina Stagh, with whom he has two children, Dan (born 1970) and Ann-Linn (born 1972) Guillou. His daughter Ann-Linn, a journalist and feminist commentator, lives in a civil union with Sandra Andersson, daughter of film director Roy Andersson.

He is now married to publisher Ann-Marie Skarp (born 1952), the daughter of colonel Åke Skarp and Märta (née Kugelberg). He has an apartment in the Östermalm district of Stockholm, where he has lived for most of his adult life. He also has a country residence in Flybo, Östhammar Municipality, northern Roslagen, where he lives when he writes his books.

Guillou started his career as a journalist writing for the magazine FIB aktuellt from 1966 to 1967. He later co-founded the Folket i Bild/Kulturfront magazine, at which he worked from 1972 until 1977. He currently writes a column for Aftonbladet and also comments occasionally in other news outlets on current events usually taking the left-wing and the Anti-American side, particularly the conflicts in the Middle East and miscellaneous domestic issues, including the United States' war on terrorism, Israeli policy towards the Palestinians, the Swedish Security Service, Swedish courtroom procedures and public inquiries.

Guillou has served as the host of several television programmes: Magazinet (1981–1984), Rekordmagazinet (last years together with Göran Skytte) and Grabbarna på Fagerhult (together with Pär Lorentzon and Leif G. W. Persson), all shown on Sveriges Television.