The thesp told The Danny Baker Show on BBC Radio 5 he would be playing Mycroft Holmes - the older brother of Sherlock Holmes.
"I'm playing Mycroft in the sequel to the Sherlock Holmes film Guy Ritchie directed with Robert Downey Jr and that sort of part is fun,".
Sources say the character in the sequel's script is described as "an agoraphobic oddball."
Fry will be joining Noomi Rapace, who played the role of Lisbeth Salander in the Swedish films of Stieg Larsson's Millennium trilogy.
UPDATE: 'Mad Mens'Jared Harris is set to play Sherlock’s number one enemy, Professor Moriarty.