The movie is being co-produced by Joe Unitas, the late quarterback's son.
He tells WBAL-TV that Flacco will portray Unitas in scenes of the 1958 NFL championship game, in which Unitas led the Colts to victory over the New York Giants. The game at Yankee Stadium ended in sudden-death overtime and has been called the greatest NFL game ever played.
Flacco, who led the Ravens to a Super Bowl victory in February and was the game's MVP, said on Twitter that he's excited by the opportunity.
Filming is scheduled to start this summer.