A 15-year-old boy suffered two stab wounds in his back during an attack in Osgathorpe Park, Pitsmoor, at around 5pm last Tuesday.
The teenager, from Sheffield, was attacked by two men and a woman, thought to have been aged between 18 and 21.
One man was white and of a stocky build, the other black and slim, and the woman spoke with a Jamaican accent.
They are believed to have had a brown dog with them.
Detectives are investigating whether the attack was 'targeted'.
They said it is not being linked with any other incidents.
A South Yorkshire Police spokeswoman said: "PCSOs have been carrying out dedicated patrols in the area to provide reassurance."
Witnesses or anyone with information should call South Yorkshire Police on 101 or Crimestoppers on 0800 555111.