Peel Regional Police said the 15-year-old male victim's injuries are possibly life-threatening.
Police said the stabbing took place around 8 p.m. at the Blue Lake Parkette, a small park with a playground in a residential area near Bovaird Drive West and Chinguacousy Road.
Police spokeswoman Lilly Fitzpatrick said the stabbing was not a random attack.
Several witnesses are being interviewed, Fitzpatrick said.
The main suspect is described only as being a black male teenager wearing a grey short-sleeved shirt, jeans and yellow shoes.
Anyone with information about what happened is being urged to contact police.