Jan Oscar 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, together with Liza Marklund and his common-law wife, publisher Ann-Marie Skarp.

Guillou's fame in Sweden was established during his time as an investigative journalist. In 1973, he and co-reporter Peter Bratt exposed a secret intelligence organization in Sweden, Informationsbyrån (IB). He is still active within journalism as a column writer for the Swedish evening tabloid Aftonbladet.

In October 2009,