The man held up the Libby store on Main Road, Ayr, on Friday evening.
He threatened staff with a weapon, which police said was not a gun, and demanded they hand over cash from the till.
He then made off with a four-figure sum of money towards Low Road.
The two women working in the shop, aged 17 and 26, were not injured but were left badly shaken by the incident, which happened at around 8.30pm.
Police said the suspect is white, aged in his early 20s, with a slim build and short dark hair.
He was wearing a black hooded jacket, dark trousers and a black scarf.
He was also wearing a medical walking boot, also known as a moon boot.
Police are appealing for witnesses as well as carrying out house to house inquiries and reviewing CCTV footage.
Detective Inspector Stephen McCulloch said: “This is a very busy area with a local chip shop nearby, I’m sure there will have been people out and about and would have noticed the man.
“I am appealing to local people to think back and consider if they saw this man, did you see him enter the store, did you see him leave?
“If you have any information, no matter how insignificant you think it is, please pass it on as it could assist our inquiries.”
Anyone with information about the incident is asked to call police on 101, or contact Crimestoppers on 0800 555 111 where information can be given anonymously.