Rachel McAdams has signed on to star in Guy Ritchie’s new take on Sir Arthur Conan Doyle’s “Sherlock Holmes,” according to the Hollywood Reporter.

McAdams is set to play the detective’s love interest Irene Adler, a character that only appeared in one of Doyle’s stories (“A Scandal in Bohemia”).

Robert Downey Jr. stars as Holmes, while Jude Law plays his assistant Watson. Mark Strong recently boarded the project to play Blackwood, the main villain in the film.

The trade says Ritchie’s project takes its cue from a forthcoming comic book that producer Lionel Wigram wrote as a selling tool for a new take on the classic character.

McAdams can next be seen in “The Lucky Ones,” which opens in limited release tomorrow. She recently wrapped filming for “The Time Traveler’s Wife” and “State of Play.”

Ritchie’s latest, “RocknRolla,” is due in theaters Oct. 31, 2008.