After what feels like months of speculation, the book's author EL James confirmed the casting news via Twitter on Monday (2 September).
EL wrote: "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. The gorgeous and talented Charlie Hunnam will be Christian Grey."
She added: "To all the supporters, lovers and haters - thank you so much for the passion that you have for this project. You all rock. All of you."
Charlie, 33, has appeared in shows including Queer As Folk and Sons of Anarchy, and recently the movie Pacific Rim, while 23-year-old Dakota's past credits include The Social Network, The Five-Year Engagement and 21 Jump Street.
She is the daughter of actress Melanie Griffith and actor Don Johnson.
Yesterday's announcement ends a wave of casting speculation that linked stars including Ryan Gosling, Cara Delevingne, Ian Somerhalder, Robert Pattinson, Mila Kunis and Matt Bomer to the movie.
Fifty Shades is a trilogy of erotic books documenting the relationship between powerful businessman Christian and naive publishing worker Anastasia.
The movie is scheduled for release in 2014.