The Chinese tourists were getting off a coach when three men tried to grab designer handbags from two women in the group.
When the 32-year-old guide intervened, the men turned their attention to him and grabbed his bag containing 20,000 euros and nearly 5,000 Swiss francs (4,500 euros) and also the watch, which he had just bought for around 5,000 euros.
The robbers then left the scene in Carrieres-sur-Seine, 20 kilometres (12 miles) west of Paris, by car.
“The guide had wanted to help two Chinese tourists targeted by three thieves who tried to take their Longchamp handbags,” a police source said.
“The guide intervened and in the end it was him who was stripped of his possessions.”
Police are investigating whether there is any link between the robbery and similar incidents in the area, including several near the Versailles palace, a popular destination for Chinese visitors to France.
France is seeing an increase in the number of Chinese tourist visitors, but their reputation for carrying large amounts of cash has made them a target of thieves.
French authorities tightened security around Chinese tourist groups in 2013 after Beijing expressed concern about the increasing number of incidents.