"It was all thanks to me," Stone told PEOPLE, laughing, of the young costars' "amazing and wonderful" romance during a lush party in the hills above Cannes, following the film's premiere.
"You could see their chemistry right away, and it was pretty powerful," the director said. "They were completely professional on the set, but there was no mistaking what was happening."
Stone was at the party along with Wall Street star Michael Douglas and Josh Brolin, who attended the soiree with his wife, actress Diane Lane.
LaBeouf, 23, and his Oscar-nominated girlfriend, Mulligan, 24, spent more than three hours together at the bash, sipping drinks and dining on smoked salmon, shrimp and special sorbets for dessert. They sat side-by-side, with LaBeouf draping an arm around Mulligan from time to time.
The two first were linked in Aug. 2009, when they were spotted dining out – and kissing – at a New York restaurant. Though neither has been especially vocal about each other – nor talked at length about their romance – LaBeouf praised Mulligan in an April interview with GQ, calling her "an unbelievably thought-provoking actress, the most talented actress I've ever met in my life, by leaps and bounds."
As for their relationship, he said at the time, "It works out. It's not like we're the premiere couple; we're not the red-carpet king and queen."
Still, Stone isn't shy about endorsing the young couple.
"I was thrilled when I realized they were going to be an item," he said. "It just made their scenes together all the better – especially the love scenes. Let's just say it was no stretch for them."