The raider threatened staff at Greggs the Bakers, in Chapelhall, Airdrie.
It took place at the Lauchope Street branch, at around 1.30pm on Monday.
He escaped on foot, with a three-figure sum of cash.
Police said no one was injured during the incident, but the two female workers were left "very distressed" by their ordeal.
The suspect is described as a white male, between 20 to 30-years-old, and 5ft 8in.
He was wearing a black beanie hat, black woollen gloves and had a black scarf covering some of his face.
Detective Constable Nicola Byrne, from Coatbridge CID, is leading the investigation.
She said: "I would appeal to anyone who saw the suspect either before or after the robbery to come forward.
"Lauchope Street is the main street through Chapelhall and may well have been busy with pedestrians and motorists at the time.
"Chapelhall is a close knit community and locals will be disgusted that this has happened to one of their own."
Anyone with information is asked to contact Coatbridge CID on 101.