Two masked men entered the Tesco Express in Finisterre Way, Littlehampton, at around 6am on Sunday, July 29.
One of the men was carrying a knife, believed to be a machete by police, and demanded the shop staff handed over money and cigarettes.
Detective Sergeant Simon Smith of Worthing Investigations said: “Luckily, the two staff members approached by these men were unhurt but they have, understandably, been left very shaken by the incident.
“The two men arrived in a white van driven by a third suspect, who remained in the vehicle during the incident.
“The man with the knife is described as white, 6’, of medium build and wearing a black bandana over his face, a black beanie hat and a dark coloured hooded top.
“The second man is described as white, 5’ 8”, of slim build, with blond hair and wearing dark coloured clothing and a white mask.
“If you saw a white van with three men acting suspiciously in this area we would like to speak to you.”
To report information please go online or call 101 quoting reference 315 of 29/07.
Alternatively, you can visit the Crimestoppers website or contact the independent charity anonymously on 0800 555 111.