Witherspoon, 36, who is expected to grow facial hair to play the titular character, will also produce the film with Peter Chernin, said the Hollywood Reporter.
The "beard" in the title refers to a member of the opposite sex, usually a woman, who acts as a date for a gay person, giving the illusion the person isn't gay.
Witherspoon is currently shooting 'Devil's Knot', Atom Egoyan's take on the West Memphis Three murder case that also stars Colin Firth. She also has the indie drama "Mud" with Matthew McConaughey. Her most recently romantic comedy was the action-filled "This Means War" with Tom Hardy and Chris Pine.