Johnson, 23 who had a small role in "The Social Network" will play the role of Anastasia Steele, while Hunnam is slated to portray Christian Grey, reported Aceshowbiz.
James who will also get involved in the project from Focus Features as a producer broke the news.
"Good morning all. Hold on to your hats. The world's top-earning novelist adds. 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," James tweeted.
"Auditions were stunning, incredible chemistry," tweets, Co-producer Michael De Luca.
The film directed by Sam Taylor-Johnson will release next year.