The incident happened at the store on Heworth Road at around 12.45pm on July 18. The suspect, who is believed to be in his 20s, entered the store and passed the cashier a note demanding money from the till.
The man then showed the member of staff what appeared to be a black hand gun in the waistband of his trousers. The contents of the till were then handed to the man and he left the store.
The suspect is described as skinny, around 5ft 10ins tall and having short blonde or ginger hair. He was wearing a khaki-coloured hoody with zip front during the incident.
Following the robbery, officers have launched an extensive investigation and are currently reviewing CCTV footage in the area and conducting door-to-door enquiries.
Detective Inspector Jackie Smart said: “Our detectives are working around the clock on this investigation but members of the public may have vital information that could help us.
“This incident happened in a relatively busy residential area at a time when people will have been out and about. So if you saw anything suspicious, no matter how insignificant it may seem, we’d like to hear from you.”
The police believe members of the public may have also seen two men shouting near the store at the time of the incident before running down Dales Lane. One continued down Dales Lane while the other turned right and ran into Harrison Street. They want to speak to anyone who might have witnessed this incident.
One man is described as 5ft 11ins tall, medium build with short dark hair, tight blue jeans and a greyish hoodie with blue and white trainers.
The other was smaller – 5ft 7ins or 5ft 8ins – medium build with short light brown hair. He wore dark blue jogging bottoms, a green cardigan with a zip and a light-coloured t-shirt underneath. Investigators have yet to establish whether the two men were involved in the robbery.
Anyone with information that could help the investigation should contact York Serious Crime Team on 01609 643279 or email SCT_York@northyorkshire.pnn.police.uk, quoting reference 12180130457.