The lone raider struck minutes after the Clothes.ie shop at the Hazelwood Shopping Centre in Glanmire, Co Cork, opened for business on Saturday morning.
According to gardaí, the man burst into the shop at 9.15am and threatened the female shop assistant with what appeared to be a kitchen knife.
He demanded the takings and she was forced to hand over a small amount of cash.
The raider then bundled the shop assistant, who is in her late 30s, into a back office and locked her inside. He also locked the front door of the shop.
The woman’s shouting alerted a passerby a few moments later and a number of gardaí arrived on the scene. However, by then the raider had fled.
The shop assistant wasn’t injured, but was said to have been very shocked by the ordeal.
Superintendent Pat Lehane, who is leading the investigation, said the man they are looking for was around 6ft tall and very athletically-built.
It is believed he had a tanned complexion, chiselled facial features and high cheek bones.
The raider was wearing a black leather jacket, blue jeans and concealed his face with a scarf. It is also believed he might have been wearing a hoodie.
Supt Lehane said the man spoke with a Cork accent and was probably in his late 30s or early 40s.
It is believed he left the scene on foot, although he may have had transport waiting for him nearby.
Detectives sealed off the area while forensic experts combed it for evidence.
Gardaí also searched the bank of a nearby river and a field by a basketball court for the knife.
Information to Glanmire Garda Station at (021) 4821002, or Cobh Garda Station at (021) 4908530.