Downey committed to the pic, which begins shooting in October, after Ritchie turned in a polish of the latest version of the script, written by Anthony Peckham.
Joel Silver, Lionel Wigram, Susan Downey and Dan Lin are producing.
Downey emerged as an action star with “Iron Man” and “Sherlock Holmes” also will take advantage of his physical skills as the character displays brawn as well as brains.
The basis for the film is Sir Arthur Conan Doyle’s classic tales, but also the comicbook “Sherlock Holmes.”
With Downey aboard, the film will go into production before a comedy that just coalesced at Columbia Pictures, with Sacha Baron Cohen playing Holmes, and Will Ferrell playing his crime-fighting partner Dr. Watson.
Downey next will be seen in “Tropic Thunder,” and then stars with Jamie Foxx in the Joe Wright-directed “The Soloist” for DreamWorks. He’ll reprise his Tony Stark role in Iron Man 2 for Marvel Studios and Paramount, with Jon Favreau directing.