Qamar Usman, 55, had just finished serving two young children at Lothian Grocers in John Street, Penicuik, when the man rushed behind the counter and held the weapon to her side.
She told the Evening News: “He came behind the counter and held a knife against the left-hand side of my stomach.
“He said, ‘I’ve got a knife, I’ve got a knife! Give me all the money’.
“I pressed the panic button and I was just thinking to myself, what can you do in this situation?
“But he kept pressing the knife against me. I was shaking and I just couldn’t do anything.
“It was a big kitchen knife, about eight or nine inches long.
“Behind me was a cigarette cabinet, I kept moving back towards it and he kept coming towards me with the knife.
“It all happened so quickly. He just kept saying he wanted the money.
“I was frightened for my life.”
The robber grabbed the till and dropped it outside the shop but managed to make off with the cash tray containing a three-figure sum. He was last seen running into Carnethy Avenue.
Mrs Usman added: “I was terrified and I’m still very shaken up. I won’t be able to rest until he has been caught.
“If I stay at home it’s going to do me more damage, I just want to carry on as normal.
“But it’s going to take a while for me to get over this.”
Mrs Usman believes the culprit, who struck on Thursday at around 4.50pm, may have shopped in the store before.
Owner of Lothian Grocers, Mohammad Munir, 53, said: “It’s shocking. My colleague has really been shaken up.
“This is the first time something like this has happened in this shop. There have been break-ins at Penicuik recently but nothing like this. We have CCTV, but I think we will be increasing our security here.”
The suspect is described as white, 20 to 30, 5ft 3in to 5ft 8in, medium build, with dark shoulder-length hair and stubble. He had a Scottish accent and wore a dark hooded top.
A police spokesman said: “Anyone who was in the area of John Street or Carnethy Road around the time of the robbery, who noticed anyone acting suspiciously in the area, should contact police.
“Likewise anyone who recognises the description of the suspect, or who has any other information that can assist our inquiries, should get in touch.”