The 20-year-old man and a 15-year-old friend were set upon by a group of youths on a football pitch at Glasgow Green.
The older of the two victims was stabbed a number of times, with police treating the incident as attempted murder.
He was taken to Glasgow Royal Infirmary, where he remains in a serious condition.
The 15-year-old suffered a minor facial injury.
Detective Sergeant Alex McArthur of Strathclyde Police appealed for witnesses to the attack, which took place at 7.20pm yesterday, to come forward.
He said: "The group who attacked the injured man were seen just before the incident near to the War memorial near to the football park and I am certain that other people in the area will have seen them and could help to identify them."