York Regional Police say they received a call about the incident at a sports centre in Vaughan on Racco Parkway at 9:45 a.m.
Shortly after noon, Sgt. Clint Whitney said that police were told that a girl had been found floating in the pool at The Pavilion.
"CPR was immediately performed on this girl, who was found by somebody here at the sports club," Whitney told reporters, noting that emergency responders arrived shortly thereafter.
Initial reports suggest the girl was under water for one to two minutes before she was found, Whitney said.
The girl was initially unconscious when found. She had no vital signs.
"Her condition is very grave, I would say life threatening. The doctors, I'm told, are doing everything possible to save her life," said Whitney.
He said the child is undergoing treatment at a local hospital, though she may soon be transported to another hospital that he did not publicly identify.
Some parents came to the sports centre to retrieve their children after the incident and have yet to speak with police. But Whitney said investigators need to make contact with them.
"We'd encourage anyone who was present, if you have children that were present and you haven't spoken to police yet, please contact us," he said.