Cops were called to the river in Bury with reports of a swimmer who had gotten into difficulties on Thursday night.
Emergency services discovered the body of a 12-year-old girl at the scene.
GMP Detective Inspector Andrew Naismith 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.”
The Met Office has predicted the hottest day of the year so far will hit this week, with temperatures to soar past 30C.
Det Ins Naismith warned people about the dangers of swimming in lakes and rivers.
He added: “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.”
Her family is being supported by specially trained officers.
A statement from GMP confirmed: “At around 7.55pm on Thursday 27 June 2019, police were called to reports of a concern for the welfare of a young person last seen in the River Irwell close to Dunster Road in Bury.
“Emergency services quickly responded and underwater search teams were sent to examine the area.
“Sadly, officers discovered the body of a 12-year-old girl.
“Specially trained officers are supporting the child’s family at this difficult time.”
The deaths of three Brits in the last 24 hours have been linked to the heatwave.
Two men died after being pulled from the sea in Torquay.
Cops declared conditions at sea in Torquay as critical after the deaths of the men, aged 25 and 26.
A man has also been seriously injured tombstoning in Torquay.