Police said a man entered a Co-op store in Malcolm Road, Tangmere at around 10.10pm last night (Tuesday, January 8) and threatened staff members with a knife.
"The man stole a quantity of money from the till and fled the store," a police spokesman said.
"He was described as wearing a light blue coloured hooded top with the hood up, light grey jogging bottoms, black trainers, black gloves and something covering his face."
Detective sergeant Graham Leaney said the suspect knew there would be limited people around as it happened late in the evening.
He added: "The staff have, understandably, been left very shaken by this incident and support is being provided.
“We would like to speak to anyone who may have witnessed the suspect either entering or leaving the store.”
Police asked anyone with any information to report it online or by calling 101 quoting 1298 of 08/01.
Alternatively, you can visit the Crimestoppers website or contact the independent charity anonymously on 0800 555 111.