The incident took place around 8.15pm on Thursday August 18 in Mini Market on New Row.
A man threatened staff before assaulting a shopkeeper and making off with a quantity of cash.
A 21-year-old man sustained minor injuries, but did not require medical treatment.
The suspect is described as white, in his forties, around 6ft 2inches tall, of medium build, with short brown hair and dark stubble.
At the time of the incident he was wearing black boots, dark jeans, a dark jacket, dark scarf, and dark peaked cap.
Detective Inspector Keith Mackay of Dalkeith CID said: “This was a very distressing incident for the staff members involved and we’re conducting numerous enquiries in the local area.
“We’d urge anyone who may have seen the suspect, or who recognises his description, to come forward as soon as possible.”
Anyone with information can contact Police Scotland on 101 or Crimestoppers anonymously on 0800 555 111.