Jan Oskar Sverre Lucien Henri Guillou (; born 17 January 1944) is a Swedish author and journalist. 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 Liza Marklund and his wife, publisher Ann-Marie Skarp.