The incident occurred around 5.30pm when the man entered the postal outlet and discreetly threatened a teller by showing a gun on his belt and ordered her to empty the contents of the till before fleeing on foot with an undisclosed amount of cash, cantonal police said.
Employees and customers in the post office were not injured, police said.
The teller threatened was a 16-year-old apprentice who was left shocked by the robbery.
Police described the culprit as a mixed African man, around 25 years old, of average build and from 180 to 185 centimetres tall.
He was wearing a white-beige hat, a dark brown pullover with long sleeves and dark blue tracksuit bottoms.
The man was carrying a gun on his belt.
The local gendarmerie and Riviera police responded to the crime scene with a dog team but were unable to immediately track down the suspect.
On the same day as the post office heist, four men linked to the robbery of 600,000 francs’ worth of valuables from a Montreux jewellery boutique on January 15th, pleaded guilty to possession of stolen goods, the ATS news agency reported.
The men, linked to the Pink Panthers gang, were apprehended in a Vienna hotel on February 5th with 25 stolen watches.
Two of the men were sentenced to two years and 21 months in jail respectively, while the other two, without criminal records, were handed suspended sentences, ATS said.