William Friedkin, who directed the film adaptation of Blatty's novel, broke the news of his death on Twitter.
"William Peter Blatty, dear friend and brother who created The Exorcist passed away yesterday," he wrote.
The Exorcist was published as a novel in 1971, with the hugely successful big screen adaptation - also written by Blatty - following two years later.
Despite a troubled production and a muted launch, the film went on to become one of the highest grossing in history and is considered by critics to be a classic horror movie.
Blatty's widow, Julie Alicia Blatty, told the Associated Press that the writer died on Thursday at a hospital in Bethesda, Maryland.
The cause of death was multiple myeloma, a form of blood cancer, she said.
Horror writer Stephen King tweeted: "RIP William Peter Blatty, who wrote the great horror novel of our time. So long, Old Bill."
The Exorcist won Blatty an Oscar for best adapted screenplay and he went on to write and direct the second sequel The Exorcist III.
Blatty also wrote novels such as Elsewhere, Dimiter, The Ninth Configuration and Demons Five, Exorcist Nothing: A Fable.
His film credits include A Shot in the Dark, The Great Bank Robbery and Promise Her Anything.
Follow us on Facebook, on Twitter @BBCNewsEnts, or on Instagram at bbcnewsents. If you have a story suggestion email entertainment.news@bbc.co.uk.
US author and filmmaker William Peter Blatty, best known for writing The Exorcist, has died at the age of 89.