Leonardo DiCaprio's heart will go on and on...for Blake Lively!

After weeks of speculation and high-profile partying up and down the French Riviera, DiCaprio, 36, and Lively, 23, were photographed holding hands during a romantic, moonlit stroll after dinner in Monte Carlo May 27-- and only Us Weekly has the exclusive photos.

"He looked very in love," a witness says in the new issue, on stands Wednesday. Two days later, the Oscar-nominated actor -- who has been wining and dining the Gossip Girl star for weeks over dinner, helicopter and yacht rides and glitzy A-list bashes -- brought her as his guest at a Grand Prix-viewing bash aboard a yacht.

PHOTOS: Check out Bauer-Griffin's photos of Blake and Leo's budding romance

"Her face lit up when Leo was talking," a fellow reveler says.

"He had introduced Blake as his girlfriend," adds a source. "I would say they're totally smitten!"

Although DiCaprio and model Bar Rafaeli called it quits just a few weeks ago, he and Lively first connected at an NYC business meeting back in November -- and shared a kiss at a Hollywood bash in January.

"I've never seen him like this with a girl," adds the insider of DiCaprio.

For much more on this suddenly hot-and-heavy romance -- how DiCaprio has spared no expense for his new lady, Lively's decade-long crush on the star, and more exclusive photos of their PDAs in Europe -- pick up the new Us Weekly, on stands Wednesday!