hotel popular with film stars, that was once the location of Alfred Hitchcock's thriller To Catch A Thief.
Armed with a semi-automatic pistol, his head covered with a cloth, he proceeded to steal jewels that were part of an exhibition by a group owned by Israeli billionaire Lev Leviev.
Authorities had initially estimated the heist was worth $53 million (€40 million), but after an inventory of the exhibition, prosecutors in Grasse near Cannes have concluded that it is valued at a much higher $136 million.
They said the robber escaped with "all kinds of different" jewellery, including "rings, pendants, and earrings incrusted with diamonds."
The heist comes head-to-head with what is considered the world's largest ever haul of jewellery - valued at around €100 million - which took place in Belgium in 2003.
Robbers at the time emptied more than 120 out of 160 safety deposit boxes inside the heavily fortified Diamond Centre in the heart of Antwerp's diamond district.
A policeman at the Carlton Intercontinental Hotel in Cannes, southern France, the scene of one of Europe's biggest jewelry heists recent years. (AP Photo)
It is also France's biggest jewellery heist, eclipsing a 2008 incident when three men stole almost every piece on display at an exhibition in Paris, a theft valued at €85 million.
"It all happened very quickly and without any violence", the prosecutor's office said Sunday about the robbery in Cannes, famous for its annual star-studded film festival.
The theft has caused anger among employees and unions, who denounced lax security at the Carlton.
The exhibition was held in a wing of the hotel with direct access to the street and police said they had not been alerted that it was taking place.
While not a legal requirement, alerting the police would ordinarily be standard procedure for luxury hotels such as the Carlton.
Prosecutors said the robber entered the exhibition room through a French window that opened onto a terrace, which itself looks onto the popular Croisette avenue in Cannes.
Police outside the hotel, from where jewels and diamonds were stolen. (AP photo)
The CGT union said on Monday that the Carlton had already been the victim of a jewellery heist in 1994, when a security guard was seriously wounded by a bullet.
According to a source close to the case, the jewellery house Leviev was using its own security guards for the ill-fated Extraordinary Diamonds exhibition, which opened on July 20.
Cannes has fallen prey to thieves several times recently, notably during this year's film festival, which attracts a glittering array of celebrities from the movie world.
A view of the Carlton hotel, in Cannes, southern France. (AP Photo)
In a pre-dawn heist at a hotel during the festival in May, thieves stole jewellery worth $1.4 million due to be loaned to movie stars.
That robbery took place in the hotel room of an American employee of Swiss jeweller Chopard while she was out for the evening, police said.
In a scene straight from a Hollywood film, a strongbox containing jewels was ripped out of the wardrobe and carried off, they said.
In a second theft during the festival, robbers made 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.