A CCTV image has been released after the incident at the 3s Convenience Store on North End Avenue, in North End, Portsmouth.
It happened at around 8.50pm on December 9, but police have only today released details.
Nothing was taken. A 26-year-old man was working in the shop when the robbery happened.
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Hampshire police said the robber ‘entered the store armed with what is believed to be a black handgun.’
A spokeswoman said: ‘Officers have conducted a number of enquiries and are now turning to the public for help in identifying the man in this image.
‘He is described as being white, around 30 years old, around 5ft 9ins tall and of a slim build. He was wearing a black jacket with a hood, and dark trousers.’
Call police on 101 quoting 44200475720 if you know the man in the photo.