Fox, who was first rumoured to be in negotiations to join the film in March, will take on an unknown role in the film.
"Megan Fox is so funny. She's hilarious. You wouldn't, I don't know if other people [expect it], but I think she's really funny, you'll be pleasantly surprised," Mann told MTV News.
Asked to reveal what part Fox would take on in the film, Mann responded: "I can't say that part."
Director Judd Apatow has said that the comedy will not be a sequel to Knocked Up but rather an entirely different movie.
Bridesmaids actress Melissa McCarthy has also signed on to star in the project as well as IT Crowd actor Chris O'Dowd.