The young woman used a handgun to threaten a worker at a branch of RS McColl in Danderhall, Midlothian, yesterday (Tue).
The raider ordered the shop assistant to put cash and cigarettes into a dark-coloured rucksack she was carrying.
Police said the robber got away with a three-figure sum of cash.
She is described as white, between 18 and 22-years-old, around 5ft 8ins tall with a slim build and cropped blonde or fair hair.
She had a gaunt appearance and pale complexion and was wearing a light-coloured cap, blue jeans, black gloves and white trainers with a red Nike tick.
The handgun is described as around 3-4 inches long and dark in colour.
A police spokesman said: “This was a terrifying ordeal for the shop assistant to have to endure and we are keen to locate the suspect as soon as possible.
“We have a detailed description and are eager to hear from anyone who believes they recognise her.
“Similarly, anyone who was in the area and has information that can assist with our enquiries should contact police immediately.”
The raid happened at the Newton Church Road branch shortly before 7pm. The attacker ran off towards the Edmonstone Road and Millerhill area.