The 17-year-old boy was hurt in what police believe was an unprovoked attack at Endcliffe Park yesterday evening.
He suffered a stab wound to his lower side and is now said to be in a stable condition.
Investigating officer Detective Constable Vicky Kenny said: “We believe that this was an unprovoked incident triggered by some sort of disagreement between two groups of young people.
“The circumstances are still being investigated but the offenders have been described as a group of around eight Asian males aged between 17 and 20.
“We are keen to hear from anyone who was in the park at the time, particularly if you were close to the wooded area behind the café where we believe the young people were taking shelter from the rain.
“We are carrying out patrols in the area and are grateful for information already received from members of the public.”
Witnesses to the incident at 7.20pm are asked to contact South Yorkshire Police on 101, quoting incident number 1080 of Thursday 30 June 2016.
Alternatively, you call Crimestoppers anonymously on 0800 555 111.