Emergency crews responded to the scene at around 6 p.m. CT. They found two people had been thrown from an ATV on a downhill slope.
The other person was a 23-year-old man from Lorne, Man., who was taken to hospital to be treated. There was no word on his condition.
The Toronto woman was pronounced dead in hospital. Her name will not be released.
Police said neither of the man or the woman were wearing helmets at the time of the crash.
Argyle is located about 200 kilometres southwest of Winnipeg.