The incident happened in the Railway Street area of Armagh.
Inspector Stephen Harvey said: “Police received a report that at approximately 9.40pm a masked male entered the premises with a suspected handgun and demanded money from staff.
“The male fled the shop with a small sum of cash. Staff and customers in the premises where not physically injured in the incident, however they were left shaken.
“The male is described as being aged in his 20s and had his face covered and was wearing gloves and dark clothing. He is described as approximately 5’11’’ tall.
“We’re appealing for anyone with information about the incident, or anyone who was in the Railway Street area at the time and saw anything suspicious to contact detectives at Lurgan Criminal Investigation Branch on the 101 non-emergency number quoting reference 1274 or 06/09/17.
“Alternatively, information can be provided anonymously through the independent charity Crimestoppers on 0800 555 111.”