Thomas B. Allen is an American author and historian. He resides in Bethesda, MD. He is also the father of science fiction writer Roger MacBride Allen. Allen is a contributing editor to National Geographic.

His most famous book to date is Possessed. It is a factual retelling of the true story of a teenage boy from Mt. Rainier Maryland who went through the rite of exorcism in 1943. Allen tracked down the sole survivor of the team that performed the exorcism, Father Walter Halloran as well as a copy of the diary kept by the team leader, Father William S. Bowdern. It was upon this case William Peter Blatty based the events of his novel, The Exorcist. First published in hardcover in 1993, the book was reissued as a revised paperback in 2000 to coincide with the release of a made-for-cable film Possessed starring Timothy Dalton as Father Bowdern. The film kept close to the facts as presented in Allen's book for the most part but did add some cinematic scares.