The incident happened around 9.40pm, at the Scotmid in the Main Street of the Deans area.
Two female members of staff were working when the suspect entered the store and brandished a knife. He then demanded cash before escaping with a three-figure sum.
The suspect is described as white, 6ft 2 ins tall, of stocky build and wearing dark jogging bottoms, a black zip jumper and a black balaclava.
A Lothian and Borders Police spokesman said: “Both shop assistants were left extremely shaken, and we are appealing to anyone who witnessed the incident to come forward. Likewise, anyone who was in the area at the time and saw the suspect, or who recognises his description, should also contact police.”
Anyone with any information can contact Lothian and Borders Police on 0131 311 3131, or the charity Crimestoppers in confidence and complete anonymity on 0800 555 111.