Staff at Maryport Post Office on High Street were threatened with a firearm this morning.
It is believed the incident happened at around 11.30am.
No one was harmed but the offender made off with cash.
Armed police officers have been deployed to the area and a search is under way for the offender.
He is described as a being approximately 6ft, aged approximately 40 to 50 years old, of slim build and was wearing a dark shiny Puffa jacket with his hood up.
High Street was closed for four hours.
Detective Inspector Dan St Quintin said: “We take all incidents involving a firearm with the utmost seriousness, and have a number of resources in the area searching for this offender.
"This is a terrible incident which has left staff very shaken, I am appealing for anyone who has any information about the incident, no matter how small, to contact Cumbria police on 101."
High Street in Maryport has been closed, while officers continue with their investigations.
Anita Renwick, 55, of High Street, was concerned for the Post Office staff.
She said: "They're nice people in the Post Office.
"I don't think anything surprises you these days.
"You just don't know who's parking up outside your house or anything."
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