That was fast.

Just weeks after announcing her split with hubby Ryan Reynolds, Scarlett Johansson, 26, has a new leading man in her life -- Academy-Award winning actor Sean Penn, 50!

A source tells the new issue of Us Weekly (on stands now) that the duo began their fling in early January when she was staying at L.A.'s Chateau Marmont while filming We Bought a Zoo with Matt Damon.

"Their first hook-up was in Scarlett's Bungalow," says the insider.

Nearly twice her age, the Milk star might seem an odd choice for Johansson, but the source points out that a common cause brought them together.

"Scarlett first reached out to Sean when she was planning to visit Haiti with Oxfam," says the source. "She knew Sean was living there in a tent and turned to him for advice."

More at usmagazine.com.