The announcement came Thursday describing Fox's character as Reagan, a "gorgeous, straight-shooting pharmaceutical sales rep who comes to town on business and shakes things when she rents out Jess' (Deschanel) room while she is sequestered on jury duty."
The Teenage Mutant Ninja Turtle star will first appear on the sixth episode of season five, which is scheduled to premiere in January.
"It's insane to me that such a gorgeous person can be so funny, but I guess we just all have to accept it," said executive producer Liz Meriwether. "I've been a fan of her comedic skills since This is 40, and I'm so excited to have her on the show. We had Megan in mind when we wrote this part – she's the perfect person to come in and shake these guys up."
In July, Deschanel and husband Jacob Pechenik welcomed a baby girl together, their first child.