“Two men pretended to be couriers to get the door opened,” a police source said.
After threatening staff there with tear gas canisters, they escaped “with about 10 watches with an estimated value of €500,000 ($546,000)”, said the source.
A spokesman for Girard-Perregaux's parent company Kering confirmed the value of the stolen watches.
In March, a Chopard jewellery store in the nearby Place Vendome was robbed of an undisclosed amount of merchandise by two men armed with a pistol and a grenade.
Security was stepped up in the chic square after a series of violent robberies there in 2014.
Both raids were in the same area that reality TV star Kim Kardashian was robbed in a luxury apartment by armed men who got away with jewellery worthm around €9 million ($10 million).