According to state forecaster AEMET a red alert is issued when extreme weather poses an “extreme risk” to people’s health.
Residents have been advised to stay out of the sun during the hottest part of the day.
On Thursday AEMET put the two Basque areas of Bizkaia and Gipuzkoa on red alert as the mercury reached 41C.
READ MORE: Europe heatwave 2019: Spain on high alert as heatwave strikes
Spain heatwave mapped
Large swathes on the country were on orange alert today as temperatures superseded 30C.
The mercury soared to 40C in Madrid, Ciudad Real, while in Jaen it reached 37C.
Friday weather warning
Temperatures are forecast to continue rising as the week draws to a close.
Tomorrow the following Spanish areas will be on red alert, according to AEMET:
In Aragon the areas of Huesca and Zaragoza will be hit by the worst on the heatwave.
Catalunas will bear the brunt of the heatwave too.
Barcelona, Gerona and Lleida will all be on red alert on Friday.
Elsewhere in the North of Spain, a red alert will be issued in Navarra.
La Rioja will also be on red alert because of the heatwave.
Speaking on Wednesday AEMET spokesman Fernando García said: “The increases will continue, with 3C or 4C more in the north of Castilla y León.”
“Friday, Saturday and Sunday will be the worst days of the heatwave, with up to 44C expected, and 42C for sure.”
Weather maps show large swathes of Spain will reach well above 30C on Friday.
These are predicted to be the hottest parts of Spain tomorrow:
Zaragoza: 43C
Valladolid: 41C
Madrid: 41C
Ciudad Real: 41C
Jaen: 40C
Seville: 37C