A man armed with a hand gun threatened staff at a shop in Windsor, in Berkshire, before escaping with cash.
Thames Valley Police said the robbery happened at the Happy Shopper store in Alexandra Road on Wednesday at about 1155 BST.
Officers said a small amount of money was stolen before the man, believed to be in his early 20s, ran off in the direction of Grove Road.
No one was injured in the incident and police are appealing for witnesses.
Det Con Iain Watkinson said: "Thankfully this sort of incident is very rare, particularly in Windsor, and no-one was injured."
The man is described as white, aged in his early twenties, about 5ft 9ins (1.75m), thin, and was wearing black jeans and a white T-shirt.