William Peter Blatty, (born January 7 1928), is a writer, probably most famous for the novel of The Exorcist and the subsequent screenplay version. He is generally respected in the horror movie community and is associated with other epic horror film writers such as Robert Bloch.

He was born in New York City to Lebanese parents but his father left home when William was six years old. Raised in relative poverty by his deeply religious Catholic mother, he apparently lived at twenty-eight different addresses during his childhood. He attended several Catholic and Jesuit schools before finding his raison d'etre and attended the George Washington University to study English.

In the 1960s Blatty formed a relationship with the director Blake Edwards by writing films such as Gunn (1967).

Screenwriting work soon ran dry, however and so Blatty took to novel writing. Allegedly retiring to a remote and rented chalet in woodland off Lake Tahoe, Blatty wrote The Exorcist, a story about a twelve-year-old girl being possessed by a powerful demon. It would eventually be translated by himself and the director William Friedkin into one of the most famous and controversial mainstream horror movies of all time. According to Blatty, parts of the screenplay were unintentionally written in an apartment with the number 666.

In 1983, he wrote a novel called Legion, a sequel to his famous first novel, the story of which later became the basis of a second film sequel to The Exorcist. Blatty originally wanted the movie version to be titled Legion but the film producers wanted it to be more closely linked to the original, and hence it was titled The Exorcist III. The other sequel, Exorcist II: The Heretic was disappointing both critically and commercially. Blatty had no involvement in this first sequel and his own follow-up ignored the events of it.

Blatty's autobiography is titled I'll Tell Them I Remember You.