"It's one of the most shocking couples we've seen in such a long time," said Jennifer Peros, a reporter for Us Weekly, which has the unlikely couple on the cover of the current issue.
"For Leo, to date this innocent actress is definitely different from his past resume of girls," Peros said. "He's always been known as this partier. But ever since he's been in Europe with Blake, they've had these quiet sit-down dinners."
After weeks of speculation that the two were getting close during the Cannes Film Festival, they were photographed May 27 holding hands while strolling down a moonlit sidewalk in Monte Carlo.
"They're having one of the most romantic European vacations ever," Peros said. "Leo's been wining and dining her along the French Riviera."
Representatives for DiCaprio and Lively did not return requests for comment.
Peros said DiCaprio is pulling out all the stops, including helicopter and yacht rides, $900 lunches and ritzy A-list parties.
"He looked very in love," a witness to the budding romance told Us.
"Her face lit up when Leo was talking," a fellow partier told the magazine.
Another source told Us that DiCaprio was introducing Lively as his girlfriend.
"I've never seen him like this with a girl," the source said.
Could this be love?
"I think it's a little too early to say he's going to settle down," Peros said. "But it should be fun to watch this cute little relationship unravel."
Fun, because DiCaprio and Lively come from two different worlds. There's the age difference, to begin with: He's 35 and she's 23.
"He feels young with her," Peros said about DiCaprio. "He likes showing her all of Europe. She's not used to all this stuff. He's teaching her about the sport of Formula One racing at the Grand Prix in Monaco. He's showing her a new world."
For Lively, dating DiCaprio may seem like a dream come true. Peros said Lively has had a crush on DiCaprio ever since she was 10 and first saw him in "Titantic."
DiCaprio quickly rose to the A-list, working with Spielberg and Scorcese, while Lively, who got her break in "The Sisterhood of the Traveling Pants" films, became the doyenne of television as the star of "Gossip Girls."
Peros said the pair first crossed paths in November at an industry meeting, then shared a kiss at a party in January. But it was Cannes, where they were both attending the film festival, that brought them together in May.
Lively's association with DiCaprio can only be good for her career, Peros said. Lively, who had a breakout role in 2010's "The Town," has been making the move to the big screen. She'll appear in "The Green Lantern" later this month.
"This [romance] is getting her more notice in the Hollywood circle," Peros said.
Already, DiCaprio has taken Lively sailing with Steven Spielberg and his wife, Kate Capshaw, aboard a yacht on the French Riviera.
While pictures from that expedition show them embracing and being affectionate, their dating track records couldn't be more different.
"Leo is notorious for being on the club scene and being swamped with models," Peros said.
In fact, DiCaprio has dated some of the world's top models, including Brazil's Gisele Bundchen, who's now married to New England Patriots quarterback Tom Brady, and Israel's Bar Refaeli, with whom he reportedly split only weeks ago.
Lively, on the other hand, hasn't been linked to anyone since ending her three-year relationship with her "Gossip Girl" co-star and former childhood classmate, Penn Badgley, in September 2010.
When rumors of nude photos of Lively surfaced days ago, her spokeswoman shot them down as "100 percent fake."
"Blake has never taken nude photos of herself," her rep told Reuters.
"She's kind of untouched in Hollywood," Peros said. "She hasn't dated anyone since Penn. So Leo gets this, 'I got her first.' She's young, she's hot. She's a fresh face."
Lively gets a multi-millionaire A-list actor whom she's always had a crush on.
Said Peros, "It's every girl's dream."