The incident was reported at around 1pm on Friday at the convenience store on Monreith Avenue in Bearsden, East Dunbartonshire.
Two men entered the shop, with one brandishing the weapon, demanding money from a member of staff.
A three-figure sum of cash was stolen.
Both men are described as being about 5ft 10in to 6ft with a slim build, and they had light-coloured masks covering their faces.
The first man was wearing dark grey Adidas tracksuit bottoms with blue stripes, a dark grey hoodie with the hood up and a blue baseball cap.
The second man had a light grey tracksuit on with the hood up.
Detective Sergeant Neil Guy said: "Nobody was injured but the staff member and a female customer were left extremely shaken.
"The men came from and headed back on to Stirling Avenue, towards the path leading to Henderland Road.
"If anyone has any information relating to this, or saw anyone either before or shortly after the incident matching the descriptions, please contact us."
Anyone with information should contact police via 101 quoting incident number 1552 of October 4, or Crimestoppers anonymously on 0800 555 111.