The incident occurred around 6.45pm on Friday night when three men, who were all wearing some form of face covering, entered the convenience store on Verdon Street, Burngreave and made demands for cash before striking the 36-year-old male shopkeeper over the head.
Police say the men, who were all thought to have been carrying knives, took a small amount of cash and fled the area on foot.
Witnesses said the culprits were last seen heading in the direction of Lopham Street.
Detective Constable Liz Kooper, the investigating officer, said: “Fortunately the shopkeeper was not seriously injured, but the level of violence in this robbery is very concerning and I would appeal to anyone who may have been in the area at the time and seen anything suspicious to come forward.
“I am particularly keen to hear from anyone who may have seen these men running in the direction of Lopham Street following the robbery.
“We believe these men were all relatively young and spoke with local accents.”
A South Yorkshire Police spokesman added: “Did you see anyone suspicious in the area? Can you help?
“Anyone with information is asked to contact police via: 101 and quoting incident 893 of the 19 February. Information can also be emailed to [email protected]”