The actor, who was recently praised for his performance in "Bareilly Ki Barfi", will join Aishwarya Rai Bachchan, Anil Kapoor and Divya Dutta in the new film.
Mehra announced the news on Twitter by welcoming Rao aboard.
"'Fanney Khan' Anil Kapoor Rajkummar Rao Divya Dutta TSeries Kriarj Aishwarya welcome aboard Raj tighten your seat belt for the roller coaster. (sic)," the director wrote.
Earlier, there were reports about Madhavan being a part of the film.
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