Emergency crews were dispatched to the Pavilion Fitness club on Racco Parkway at 9:45 a.m. after the girl was found floating in the pool.
The girl was initially unconscious when she was found and had no vital signs.
Initial reports suggest the girl was under water for one to two minutes before she rescued, York Regional Police Sgt. Clint Whitney said on Tuesday, adding that CPR was performed on the girl before emergency responders arrived.
The girl was transported to hospital and had been breathing on her own in stable but critical condition before police announced her death today.