The attackhappened around 10pm on Thursday, in the Pollok area of Glasgow.
Police said the victim was in the playpark in Brockburn Road, near Levernside Crescent, when he became involved in an argument.
The other youth stabbed the boy, then ran off towards the Crookston Castle area.
The victim was taken by ambulance to the city's Southern General Hospital.
The culprit is described as being white, about 15-16 and 5ft 5ins. He is slim, with stretched earrings.
Detective Constable Lauren Docherty, of Greater Glasgow CID, said: "Although we've managed to speak to some of the youths in the park at this time, I believe there are many more we have to speak to."
Call police on 101 or Crimestoppers on 0800 555111.