Credit: John Shearer/Getty Images; Jon Kopaloff/FilmMagic

Scarlett Johansson is so over Ryan Reynolds.

The actress, 26, and new love Sean Penn, 50, kicked off a steamy 24-hour getaway at the Las Ventanas al Paraiso resort in Cabo San Lucas, Mexico, March 2.

"They were acting like teenagers in love and were all over each other," a source tells the new issue of Us Weekly (on stands now). "Kissing a lot and chain smoking."

PHOTOS: Odd couples

After retreating to their $3,860-a-night ocean-front suite, the handsy duo didn't emerge until 2:00 the following afternoon for a liquor-loaded lunch at the hotel's Sea Grill (three margaritas and a shrimp cocktail for her, three pina coladas and lobster for him).

"She's head over heels," a source says of Johansson.

The twosome then returned to their room for three more hours of alone time before boarding a private jet to L.A.

For more details on their steamy tryst, pick up the new issue of Us Weekly, on stands now!