Sean Penn Flaunts New Girlfriend, Sexy Muscles

It's official: Sean Penn and Scarlett Johansson are off, and suddenly it's apparent why she was so devastated. Not only has Penn locked down another young love, he's also showing off an unbelievably fit 50-year-old body.Penn: 1, Johansson: 0. (Make that negative one -- Johansson's divorce from Ryan Reynolds' became finalized last Friday.)

There's no question that Sean Penn celebrated the Fourth of July with some heat and fireworks: two weeks after first being spotted with Stacey Koplin, yesterday, Penn was snapped on an early-morning run with her in Malibu and, later in the day, photogs caught them dressed casually and heading to a party at a pal's place before being spotted on the beach and climbing cozily onto a jet ski.

So, who is this mystery girl? Not much is known about her yet, but she seemed to get a kick out of photographers' interest in her and her shockingly-buff boyfriend.(Though, later as Penn helped hoist his new lady onto a jet ski, she appeared to shield her face from the paps.)

Clearly Koplin is comfortable with a love affair that's a little lower-profile than the PDA-filled relationship that Penn and Johansson enjoyed last spring...but is the Oscar-winning actor really ready for something long-term? His 20-year relationship with Robin Wright ended in divorce in July 2010, and it looks like Penn wants to ring in the first anniversary of bachelorhood with a bang. We just hope his new interest is still smiling in a month: word has it that Penn can be unflatteringly critical of the women he romances.

Still, no matter how stressed Penn looks in the face these days, his stunning shape illustrates why this Hollywood bad boy continues to get his way with the ladies. Flip through our Sean Penn slideshow, and hit the blue links for more.