He was assaulted on the corner of Castle Square and Arundel Gate, close to Poundland, at around 10pm yesterday.
The man was taken to hospital with serious head injuries and a police cordon was put in place around the crime scene while officers pieced together exactly what happened.
South Yorkshire Police said: “Last night, we received reports of a man injured following an assault by another man on the corner of Arundel Gate and Castle Square.
“Our officers attended the scene where they performed initial first aid and erected a cordon to conduct their investigation.
“The victim was taken to hospital by ambulance, where he is currently being treated for serious head injuries.”
No arrests have yet been made.
Anyone with information should call South Yorkshire Polie on 101 and quote incident 898 of June 23.