Just before 10pm two men entered the shop armed with kitchen knives and demanded money from the till. The men stole cash before running out of the shop.
Both men ran across the road, in the direction of BransburyPark, causing traffic to stop. The men were then seen to go into the park.
It is thought that the men ran through the park to a waiting car.
Both of the men were described as medium build; one was wearing grey trousers and white trainers, while the other man was wearing a dark-coloured top.
DS Barry Ward said: “This type of robbery is not common and fortunately no one was hurt; however the shop worker would have been shaken by this incident.
“We take all incidents involving weapons extremely seriously and we’ve got a dedicated team of officers working on this case to find the men responsible.
“We know that the men ran out into the road and stopped traffic; therefore a number of people would have seen the men. We are appealing for those people to come forward and tell us what they saw.”
A 25-year-old woman from Southsea has been arrested on suspicion of robbery and use a motor vehicle on a road / public place without third party insurance, in connection with the incident.