The expensive piece by Swiss jeweller De Grisogono vanished after a celebrity party at a five-star hotel in the resort town of Cap d’Antibes.
The theft occurred despite “large security measures”, including 80 security guards, the jeweller said.
Last week, thieves ripped a safe with jewels from the wall of a hotel room.
The latest robbery happened after an exclusive gala held at the exclusive Hotel du Cap-Eden-Roc on Tuesday night.
The high-profile event celebrated De Grisogono’s 20th anniversary, and was attended by international movie stars like Sharon Stone and Ornella Muti.
The jeweller house regularly loans its jewels to celebrities, including Paris Hilton and Cameron Diaz.
“It is actually the first time it has happened in our 20-year history,” the company said in a statement.
It added that the theft occurred “despite the large security measures set in place: over 80 security guards plus police”.
Last Friday, thieves stole jewels by Swiss jewellers Chopard from the Novotel hotel.
The precious goods were worth more than 770,000 euros.
Chopard is an official sponsor of the festival and also makes its top award, the Palme d’Or. But the trophy was not among the stolen items.
At least two apartments rented by film executives have also been burgled during the festival, according to the AFP news agency.