The "Juno" director is planning to assemble a cast of A-list actors to honour the film, which won the People's Choice Award at the Canadian festival in 1987.
"In 1987, Princess Bride won Toronto Film Festival. On Sep 12th, we're going to pay tribute to that perfect script," he wrote on Twitter.
The original film, about a man's fantasy journey to save his long-lost love, starred Cary Elwes and Robin Wright.
Reitman recently directed a table read of "The Big Lebowski", starring Jennifer Lawrence and Michael Fassbender.