Jan Oskar Sverre Lucien Henri Guillou (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 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.