South Yorkshire Police, who are appealing for information about the attack, say the man attended hospital at 1pm today “with stab wounds to his body”.
The force said in a statement: “It was reported that he had been assaulted on the tram bridge over the Park Square roundabout.
“The man, from the Sheffield area, is in a serious condition in hospital where he is undergoing treatment.
“Officers would like to speak to anyone with information about the assault or who was in the area between 11am and 1pm.”
Anyone with information should call South Yorkshire Police on 101 quoting incident number 667 of 23 August 2015.