Law is best known for his roles in "The Talented Mr. Ripley," "Alfie" and "Cold Mountain."
Ritchie's credits include "Lock, Stock and Two Smoking Barrels," "Snatch" and the current release "RocknRolla."
The BBC said Ritchie and producer Joel Silver said their new adventure movie based on the mysteries of Arthur Conan Doyle will be shot in London for three months at locations such as St. Paul's Cathedral and Freemasons' Hall.
"I'd like to bring something new to this project -- I have a few tricks up my sleeve. It won't be a traditional Guy Ritchie thing, it will be a new Guy Ritchie thing," the BBC quoted Ritchie as saying.
"I think me and Guy are well-suited to working together," Downey said. "The more I look into the books, the more fantastic it becomes. Holmes is such a weirdo."