The woman was working at Auchenharvie filling station in Boglemart Street at about 07:45 when a man came in with a knife and demanded money.
He fled with a two-figure sum of money, heading towards Aisla Street.
He was wearing a navy cap with black Nike lettering, a black Berghaus rain jacket, and white trainers. He was carrying a yellow JD Sports bag.
Det Insp Mick Carr, of Ayrshire CID, said: "Our inquiries are under way to trace the man responsible for this robbery. The shop attendant was not injured, but was left in a state of shock following the incident.
"We've conducted door-to-door inquiries and continue to assess CCTV to determine his direction of travel following the robbery.
"I would ask anyone with any information to get in contact. You may have seen a man in dark clothing running towards through the houses behind the petrol station. You may not think the information you have is much, but as part of our broader investigation, it could really make all the difference."
A 63-year-old woman working at a petrol station in Stevenston has been threatened by a man with a knife.