The robbers - who were believed to be two men - raided Iceland, in High Street, Cradley Heath, at around 5.30pm on Friday.
West Midlands Police are investigating the terrifying incident, which left staff with minor injuries.
A spokesman for Iceland said: "Two males entered our Cradley Heath store. One put a knife to the throat of a member of staff, and instructed him to take them to the office, where they told him to open the safe.
"When he took too long to do this one of the males punched him.
"The safe was opened and they made off with approximately £1,500 in coins.
"The males left in a car, the registration number of which has been given to the police."
The spokesman added: "It is naturally distressing and depressing that this sort of criminal activity continues against the backdrop of the Covid pandemic."
West Midlands Police has appealed for anyone with information to contact officers.
A force spokesman said: "Two suspects entered the shop armed with knives at around 5.30pm.
"They threatened staff members, and assaulted two of them, leaving them with minor injuries.
"Staff were forced to open the safe and the suspects made off with cash.
"An investigation is ongoing and anyone with information should get in touch."
Anyone with information is asked to contact police, by visiting www.west-midlands.police.uk and using the live chat tool, quoting crime reference number 20SW/14661B/21.
Alternatively, contact Crimestoppers anonymously on 0800 555111 or at crimestoppers-uk.org