The alarm was raised at around 7pm on Saturday after an 18-year-old male walked in to the Gala shop on Pope’s Quay.
He threatened staff with a knife and fled on foot with about €150 in cash.
Gardaí, who were on patrol in the area, spotted the youth running along the North Mall.
They pursued him and arrested him under Section 4 of the Criminal Justice Act and took him to Mayfield Garda Station for questioning.
The youth, who has an address in north Cork, was released without charge before midnight.
Nobody was injured in the incident but shop staff were said to have been badly shaken by the ordeal.
A file is being prepared for the Director of Public Prosecutions.
It is the second robbery of a shop in that part of the city in the less than a week.
Last Tuesday, a man wielding an iron bar burst into the Centra Store on nearby Coburg Street at about 10.45pm. Three male staff members, all aged in their 20s, were on duty.
The raider demanded money, smashed the cash register with the bar and grabbed a significant amount of cash. But as he attempted to get away, the three staff members tackled him to the ground outside the store.
One Polish shop worker sustained a minor head injury and was treated in hospital.