Peterborough police said the stabbing was reported in the area of King Street South and George Street North at around 11:30 p.m. on Saturday.
Police said a group of friends were walking on the street when an unknown man approached them. The suspect stabbed one of the males in the back, police said.
The victim was taken to Peterborough Regional Health Centre for non-life-threatening injuries, police said Monday.
The suspect was described to police as having a medium build with dirty blonde, shaggy hair.
Police deployed the K9 unit, but did not locate a suspect.