The incident took place at about 11.45am. Officers were called to Millbarn Furniture shop, in the Windmill Shopping Village, Yapton Road, Barnham.
A few minutes beforehand the 59-year-old woman working there was threatened by a man wearing a balaclava who demanded money.
A spokesman for Sussex Police said: “The man then pushed her by her shoulders and hit her on the side of her face with an object of some kind. He then ran out of the shop empty handed, and turned right as though to go to some nearby fields.
“The assistant was shaken and sustained bruising to her face.”
Officers are now hunting the suspect and are appealing for help in finding him.
The man was described as black, in his 30s, 5ft 10ins, of medium to heavy build, wearing all black ‘fitted’ clothing and black ‘fitted’ gloves.
Police searched the area but did not find the man.
Detective Constable Paul Gilmour said: “If you were in or around Barnham that morning and saw this man, or if you have any other information, please contact us via [email protected] or call us on 101, quoting serial 612 of 23/06.
“You can also call the independent charity Crimestoppers anonymously on 0800 555 111.”