A man armed with a gun entered a branch of BCBE, the Bern cantonal bank, in the town of Malleray on Monday at around 8.55am, cantonal police said.
He demanded money from a teller, threatening her with the gun, police said.
She had initially refused before giving him an undetermined sum.
The robber then fled in an unknown direction.
No-one was injured in the hold-up.
Despite a significant search operation, police said they were unable to track down the culprit.
Investigators have clues to the man’s identity but police did not disclose any information.
The cantonal force issued an appeal to the public for anyone with possible information about the incident to contact them at 032 344 51 11.
Two previous break-ins at the bank earlier this year remain unsolved.
On April 25th, a man made off with an undisclosed amount of money from the branch, which is located not far from the French border.
A month later, police believe the same robber, armed with a handgun, seized foreign money worth tens of thousands of francs from the bank.
The man — never arrested — was described as between 20 and 30 years old, speaking French with a foreign accent and about 180 centimetres tall.