Toronto police said it happened around 3:36 a.m. Sunday in the Kipling Avenue and Mount Olive Drive area, just north of Finch Avenue West.
Police said there was an altercation between a group of people in the parking lot of a park and a woman was stabbed.
She was taken to hospital and is now listed as being in life-threatening condition.
Police said a man was arrested in connection with the incident.
Anyone with information was asked to contact investigators.