A man threatened staff with an axe at a shop in Oldham, before stealing tens of thousands of pounds in cash.
The man, wearing a balaclava, followed a security guard into Tesco Express in Lees Road at about 1415 BST on Tuesday.
He followed the guard into the office, threatened him and another member of staff with an axe and ordered they hand over a cash box, police said.
The man, described as white, 6ft (1.8m), and of slim build, left in a silver Audi car.
He was wearing a grey hooded top with three stripes on the arm, black trousers and black gloves
Det Sgt Jason Byrne, of Greater Manchester Police, said: "Both the staff member and security guard were afraid what this man may have done and he was clearly determined to get what he wanted.
"I would urge anyone who may have seen this man enter the store or witness the silver Audi either pulling up at the store or driving off to contact us."