Director Ridley Scott recently suggested that the film, which he described as "kinda R", could be edited by execs in order to ensure a more lucrative PG-13 rating.
However, 20th Century Fox CEO Tom Rothman has told Collider that the movie "will not be compromised either way" and that the studio will accept whatever rating the MPAA gives the film.
A fan has now posted a picture of a pre-sale ticket for the movie on IMDb, suggesting that its official rating is R.
Prometheus, which stars Noomi Rapace, Charlize Theron, Idris Elba and Michael Fassbender, tells the story of explorers on an alien planet.
The film marks Ridley Scott's first foray into science fiction since Blade Runner.
Two viral videos, one featuring Fassbender as android David, have been released in promotion for the film.
Guy Pearce, who plays business magnate Peter Weyland, has said that more are on the way.
Prometheus is scheduled for release on June 1 in the UK and June 8 in the US.
Watch Guy Pearce as Peter Weyland in the first viral video for Prometheus below: