The directors confirmed the rumoured film name on Twitter.
"You also might be interested to know that we are almost done with the Dumb and Dumber To (sic) script," they wrote. "I think you're going to like it."
Jim Carrey and Jeff Daniels are both reported to be on board to reprise their roles in the follow-up to the 1994 comedy.
The sequel centres around one of the duo seeking out their long-lost child for a kidney transplant.
Dumb and Dumber To is scheduled to begin shooting in spring 2013, working around Daniels's commitment to HBO drama The Newsroom.
The 2003 prequel Dumb and Dumberer: When Harry Met Lloyd was made without the involvement of the Farrellys or the original stars.
Photo gallery: Dumb & Dumber in pictures:
Copyright: Rex Features Moviestore Collection