The Film Stage confirms that the project, in which Phoenix plays a man who falls in love with the voice of a computer, will be called Her.
Mara replaced Carey Mulligan this spring, after a scheduling conflict forced the British actress to drop out, while Samantha Morton, Olivia Wilde and Amy Adams also star.
The official plot synopsis for Her is as follows:
"In the not-so-distant future, Theodore (Joaquin Phoenix), a lonely writer, purchases a newly developed operating system designed to meet the user's every needs. To Theodore's surprise, a romantic relationship develops between him and his operating system.
"This unconventional love story blends science fiction and romance in a sweet tale that explores the nature of love and the ways that technology isolates and connects us all."
Watch Rooney Mara speaking to Digital Spy about The Girl with the Dragon Tattoo below: