A teenager spent two days in hospital and had to undergo surgery after being assaulted in Lowton.
At about 9.45pm on 2 May 2012, an 18-year-old man was on Newton Road waiting for a friend, when he was approached by a group of four youths: three boys and a girl.
The victim went to walk away but was set upon in an unprovoked assault.
He was punched in the face and had his head stamped on as he lay on the ground.
One offender tried to snatch his phone from his hand but he managed to keep hold of it.
A woman in a passing car intervened and the offenders ran off.
The victim was taken to hospital for treatment to a fractured jaw and cuts.
The offenders are described as white and being between 15 and 18 years old.
Detective Constable Matt Higham, of Wigan CID, said: “I am appalled that these offenders have been willing to stamp on the head of a defenceless man lying injured on the ground. Their actions were nothing short of disgusting and cowardly.
“Thankfully a Good Samaritan intervened, but had she not the victim could have suffered far worse injuries.
“If anyone has any information that can help I would urge them to get in touch.”
Anyone with information is asked to call Wigan CID on 0161 856 7226, or Crimestoppers, on 0800 555 111.