Hunnam, best known for his role as an outlaw biker on "Sons of Anarchy," will play Christen Grey, a suave young billionaire with a personal helicopter and bondage fetish.
Johnson -- the relatively unknown 23-year-old daughter of Don Johnson and Melanie Griffith -- will play the naive and sexually inexperienced Anastasia Steele. The actress previously starred in FOX's short-lived sitcom, "Ben and Kate."
"50 Shades" author E.L. James made the casting announcements on Twitter Monday.
The gorgeous and talented Charlie Hunnam will be Christian Grey in the film adaptation of Fifty Shades of Grey.— E L James (@E_L_James) September 2, 2013
I am delighted to let you know that the lovely Dakota Johnson has agreed to be our Anastasia in the film adaptation of Fifty Shades of Grey.— E L James (@E_L_James) September 2, 2013
"They loved her for the role," the source said. "She was thrilled to be in the running and is very excited."
Sam Taylor-Johnson, who directed the 2009 John Lennon flick "Nowhere Boy," will helm the project.