Officers are appealing for information after the McColls convenience store next the junction of Atherton Road and Palin Street in Hindley was robbed on Thursday 19 November 2020.
At around 6am, three men - who were wielding sledgehammers and a wooden fence post - entered the store and threatened staff, demanding they open the safe and tills.
No one was injured during the robbery, but staff members were left understandably shaken.
The offenders fled in a white van with a quantity of cash and cigarettes.
They are described as three white men who were all wearing dark clothing.
Detective Constable Sally-Anne Skellern, of GMP's Wigan district, said: "These men were willing to threaten innocent people with weapons, and although no one was injured, the victims were left feeling frightened and understandably upset.
"We are appealing for anyone who may have been in the area at the time and witnessed this incident or have seen anything suspicious to please come forward and speak to police. Even the smallest piece of information could help officers to complete their enquiries."
Anyone who has any information about the robbery should call police on 0161 856 7770 quoting incident number 402 of 19/11/20.
Details can also be passed on anonymously to the independent charity Crimestoppers on 0800 555 111.