See Justin Theroux's Sexy Six-Pack Abs!

Justin Theroux in Capri, June 2012. Credit: Grifoni-Sarmiento-Splash News

Tropic Thunder, indeed!

During the sun-filled last leg of his European tour with Jennifer Aniston, screenwriter/actor Justin Theroux, 40, went shirtless and revealed his sexy six-pack abs in Capri June 18. "Justin runs a lot" to stay in shape, a pal tells the new issue of Us Weekly of Theroux's stealthily chiseled body.

A dedication to fitness is just one of the many things Theroux and Aniston, 43, have in common. While staying in a $3,750-a-night room at JK Place in Marina Grande, the two frequently visited the hotel's gym, spa and pool.

Tattooed Theroux -- who kicked off his vacation with Aniston in Paris June 11 -- "sunbathed but didn't swim" aboard the $17,555-per-week Liberty of Lymington yacht the following day.

For more exclusive photos from the couple's intimate vacation -- and new details about their plans to wed -- pick up the new issue of Us Weekly, out Friday!