The incident happened around 9.30pm yesterday at the Scotmid shop in Corslet Place, Currie.
Two men aged 43 and 18 were working inside when a man entered and presented a knife before demanding money.
A three-figure sum was then stolen from the store before he ran off..
Police are now urging anyone who can assist with their enquiries to come forward.
The raider is described as white, between 35 and 45-years-old, 6ft tall with a skinny build and short cropped brown hair that was receding at the front. He was wearing a brown t-shirt and dark bottoms.
Detective Constable Marion Fraser said: “Neither member of staff was injured during this incident, however we are keen to trace the suspect as soon as possible and would ask members of the public to get in touch if they remember seeing any suspicious activity in or around the Corslet Place area last night..
“Anyone who can help us identify the male responsible for this robbery is also asked to contact police immediately.”
Those with information can contact Police Scotland on 101 or alternatively, the charity Crimestoppers on 0800 555 111.