Jen Aniston "Diving in Headfirst" With New Man Justin Theroux

Credit: Kevin Mazur/; Barry King/FilmMagic

Undercover lovers!

Jen Aniston is trying to keep it on the DL, but she's secretly dating Justin Theroux.

"Jen is totally into him," a source tells Us Weekly. "They have been hanging out for months."

The two started off as pals and costars on the set of Wanderlust. But lately Theroux, 39, and Aniston, 42, are inseparable.

"She wants to be with him all the time," the source adds. "She's diving in headfirst." Aniston hasn't dated anyone seriously since John Mayer in 2009.

Tell Us: Do Jennifer and Justin make a cute couple?