Two men entered the store, in Findon Road at 8.20pm, and threatened those inside with handguns.
The men stole several hundred pounds in cash, together with a mobile phone belonging to the member of staff.
One of the robbers demanded that the sole customer, a 29-year-old man, turn out his pockets, and when the man refused the robber struck him on the hand with the gun causing a cut for which the victim was treated at Worthing Hospital.
No shots were fired and the men ran out. No vehicle was seen.
Both suspects are described as white.
One was 5 ft 8 ins - 5 ft 10 ins, of medium build, wearing dark trousers and shoes, and a casual top with a scarf or similar covering his face.
The second was 6 ft- 6 ft 2 ins, of stocky build, wearing blue jeans, dark shoes and a dark top. He too had clothing concealing his face.
It is believed both suspects were wearing gloves.
Detective Constable Kirsty Murray said; “If you were in the area and saw these two men, either on foot or perhaps with a vehicle, please contact us via [email protected] or call 101, quoting serial 1561 of 5/3. You can also call the independent charity Crimestoppers anonymously on 0800 555 111.