The robber entered the Scotmid on Macphail Drive, New Farm Loch at around 9.20 pm on Monday.
He had a knife and instructed staff to move before stealing a three-figure sum of cash by forcing open the tills.
The suspect is described as white, aged mid to late 20s, around 6ft tall with a slim build. He was wearing a purple hooded top with the hood up and a scarf over his face, dark tracksuit bottoms with three white stripes and dark trainers.
The suspect was last seen heading in the direction of MacEwan Place.
Detective Sergeant Peter Sweeney, of East Ayrshire CID, said: “Nobody was injured and our inquiries are ongoing to trace the person responsible.
“If anyone saw this man either before or after the robbery took place, or has any information about this incident, please contact us on 101 quoting incident number 3990 of 7 October 2019 or anonymously via Crimestoppers on 0800 555 111.”