The shaken uncle of the two-year-old girl said she had briefly escaped the supervision of family members before she was found floating in the pool.
"It happened so fast," said Francois Dufault. "I was on the phone, everyone was very stressed and started yelling, 'call 911, call 911' and I did before giving the phone to the child's mother."
Bruce Dyotte, a spokesman for the local ambulance service, said family members were already adminstering cardiopulmonary resuscitation when emergency crews arrived.
Paramedics took over and tried to use a defibrillator on the girl's heart but to no avail.
The child was rushed to hospital as attempts to revive her continued but she was declared dead shortly after arrival.
Her mother was also taken to hospital suffering from nervous shock.
Police are investigating the incident.
The drowning came just four days after an 18-month-old girl suffered the same fate and a week after a 16-month-old girl died in similar circumstances.
Thursday's death occurred in an above-ground pool in Saint-Remi, 30 kilometres south of Montreal.
There have been more than 40 drowning deaths in Quebec this year.