Paramedics raced to a Montreal-area home on Wednesday to try and revive the 16-month-old child but say she had gone into cardiac arrest.
The girl's death was the 40th drowning recorded in Quebec since the beginning of the year, says the Quebec Lifesaving Society.
Stephane Smith, a spokesman for the Urgences sante ambulance service, said emergency crews were sent to the suburban Kirkland home after a morning call from the child's mother.
"The woman was with her daughter," Smith said as he recounted the chain of events. "The little girl was watching television. The woman went upstairs to fix something on the second floor.
"She came back a maximum of one minute later and realized the little girl had gotten through the patio doors."
The pool, which had no fence, lay beyond the unlocked doors and Smith said the child fell in and drowned.
The mother called 911 and paramedics immediately went to work on the child even though there were no signs of life.
"They tried to revive her but unfortunately it didn't work," Smith said. "They transported her and continued the efforts during transport."
A team at the hospital also tried but the infant was declared dead shortly afterward.
Smith said the parents were treated for shock. The mother was home alone with the child and the father was called from work.
Const. Anie Lemieux, a Montreal police spokeswoman, said police are treating the death as an accident but are investigating to determine the exact circumstances.
The Quebec Lifesaving Society says the 40 drownings in the province, which have taken place in a variety of places such as rivers and lakes as well as pools, are 15 more than at the same time last year.
Raynald Hawkins, the society's director general, attributed the rise to warm weather that is drawing more people into the water.
"That increases the probability of incidents and accidents that happen," he said. "That's the only way to explain it.
Hawkins said July and August are prime months for drownings in Quebec because of the weather and urged people to heed security precautions. While tougher safety rules were imposed on pools in 2010, he said it's hard to determine whether older pools have been brought into line.
In Quebec City, Health Minister Yves Bolduc urged people to be careful when going in the water.
"It's true we have more drownings this summer," he said. "It's worrying when you look at it."
Bolduc is in favour of children being taught water safety as soon as possible. He also said adults have a responsibility.
"It's a question of surveillance, it's a question of protection," he said.
"There's a responsibility to supervise our children, to put in place measures to protect them around the pool as well as giving the young training to understand the risks and give them the means to avoid drowning," Bolduc said. "That would be a good thing."
Smith was straightforward in his advice.
"Be careful, be careful, be careful," he said. "Take all the precautions possible."
— Compiled by Nelson Wyatt.