Before a peak in the August hot spell expected in the second half of the week, the nationwide average temperature over 24 hours hit 26.9°C, Meteo France announced Wednesday.
Such continuously high temperatures have never been recorded so late in summer, the weather bureau said.
Monday had already brought temperatures topping the previous 26.6°C record set in 2012.
Nineteen of mainland France's 96 departments were on red alert for heat on Wednesday, with 37 others at the lower orange warning level.
"A lasting and intense heatwave is in place across the southern two thirds of the country," Meteo France said.
Maximum temperatures in southern regions including the Rhone valley, the Occitanie region bordering Spain and the Aquitaine region around Bordeaux could reach 42°C, the authority said.
Southern France wildfire - in pictures
Meteo France said much of the south would see temperatures coming close to or touching 40°C on Thursday, with minimum temperatures passing 20°C.
While the south is bearing the brunt of the heatwave, the north is not escaping weather-related misery altogether.
Parts of the north-west – although not Brittany – were being placed on orange alert from midnight on Wednesday for risk of severe gales accompanied by hailstorms and heavy rain.
Meteo France warned of wind gusts reaching 100kph.