Author EL James confirmed via Twitter that Johnson will play innocent college graduate Anastasia Steele in Sam Taylor-Wood's adaptation of her erotic novel.
"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 wrote.
James's bestseller sees the virginal Ana being seduced by mysterious and kinky billionaire Christian Grey, and has been adapted for the big screen by Saving Mr Banks writer Kelly Marcel.
The role of Christian Grey is yet to be cast, but rumoured contenders have included Garrett Hedlund, Ian Somerhalder, Henry Cavill and Stephen Amell.
Johnson is best known for her role in short-lived Fox sitcom Ben and Kate, and her supporting role in David Fincher's The Social Network.
> 'Fifty Shades of Grey': Everything you need to know about the erotic series
Fifty Shades of Grey has been confirmed to be released on August 1, 2014 in US cinemas, with international details to be revealed soon.
Photo gallery - Dakota Johnson in pictures: