Greater Manchester Police said the girl was found by underwater search teams after officers were called to the River Irwell in Bury just before 8pm on Thursday.
With the country set to see the hottest day of the year so far this weekend, police issued a warning about the dangers of cooling off in water.
Detective Inspector Andrew Naismith, of GMP's Bury district, said: "This is an incredibly tragic incident in which a young girl has lost her life, and my thoughts are with her family at this devastating time.
"We have a team of detectives working on this, but there are not believed to be any suspicious circumstances at this time.
"With the warmer weather, it's tempting to go into the water to cool off, but I'd like to remind everyone of the dangers of playing near or swimming in rivers, lakes and reservoirs and would strongly urge against this."
This weekend, short hot spell will see the mercury climb across the country, mirroring the heatwave blasting much of Europe.
According to the Met Office, the temperature could reach a maximum of 34C (93.2F) in London and the east of England on Saturday.
The hottest conditions recorded in 2019 so far were at Weybourne in Norfolk on June 2, where the temperature reached 28.8C (84F).
The highest ever temperature on record for June was 35.6C (96F) in 1976.
Across Europe, hot Saharan winds have brought scorching weather with temperatures in some parts exceeding 40C (104F).
Meteorologists put more than half of France on alert for high temperatures, while in Germany rescue services urged people to look out for young children, the elderly and others at risk in hot conditions.
Summing up the UK weather for Friday, Greg Dewhurst, a Met Office meteorologist said: "It's a bit of a cloudy start across some eastern parts of the country but this will soon clear and we're looking at a sunny day for much of the UK."
Mr Dewhurst said weather conditions will see an "east west split" with the highest temperatures in the west and an easterly breeze cooling the other half of the country.
Temperatures in Wales and the west country, including festival-goers at Glastonbury, could see temperatures of 30 or 31C on Friday.
In west Scotland it could reach 27 or 28C and in Northern Ireland 25C, while in Aberdeen it will be a cooler 17C and in London and down the east coast 24C.
After a humid Friday night, Mr Dewhurst said on Saturday the highest temperatures will be in the east, with a maximum of 34C possible in an area covering London and up towards Lincolnshire.
"We will see the hottest day of the year so far," said Mr Dewhurst.
He said the spike in UK temperatures was not a heatwave, which are typically considered to last three days or more.
Mr Dewhurst said the outlook for next week was a mix of sunny spells, scattered showers and lower temperatures.
By Eleanor Barlow and Tom Pilgrim, PA