Two thieves stole a 70,000 euro (NZ$117,000) diamond ring from a Cannes jewellery store, pocketing the gem while a saleslady had her back turned, a police source said.
Police in the French Riviera resort town - which has been hit by a series of spectacular jewellery heists this year - said the men had eastern European accents.
They pilfered the ring from a jewellery store near the luxury Gray d'Albion hotel.
Police have opened an investigation and said they will examine security videos for evidence.
On July 28, an armed man pulled off a brazen heist in broad daylight at the luxury Carlton Hotel in the town, in one of the world's biggest jewellery heists.
Brandishing a semi-automatic pistol, his head covered with a cloth, the robber stole jewels that were part of an exhibition by a group owned by Israeli billionaire Lev Leviev.
French authorities said he escaped with all kinds of jewellery, including rings, pendants, and diamond-encrusted earrings, worth an estimated 103 million euros.
Cannes has fallen prey to other jewel thefts several times recently, and during this year's film festival in May.
At the festival, which attracts a glittering array of celebrities, thieves stole jewellery worth $1.4 million due to be loaned to movie stars.
Yet another theft during the festival saw robbers make off with a diamond necklace with an estimated value of $1.9 million.
At least two apartments rented by film executives were also burgled during the 2013 festival, with thieves taking cash, jewellery and other personal items.