"Today we mourn the death of another member of the team fighting fires in Leon," Nicanor Sen, the national government's representative in Castile and Leon wrote on X.
Local media identified the man as 37-year-old Jaime Aparicio, who suffered burns to 85 per cent of his body after being trapped alongside another volunteer near the town of Nogarejas as they attempted to create firebreaks with brush cutters. He was the seventh fatality linked to wildfires this year.
His colleague Abel Ramos, 35, died on Tuesday. On Monday, a separate fire killed a man on the outskirts of Madrid.
Police said they arrested a man in northern Zamora province for starting a fire on a plot of land used to dump rubbish illegally.
The hot weather caused it to spread rapidly, burning 4000ha and leaving five people injured, they said.
Another man was arrested for provoking six fires earlier in August in the southern province of Malaga, police said on Thursday.
Ten people have been arrested since June 1, said police, who were investigating a further 38 for deliberately starting forest fires. Convicted arsonists in Spain could face up to five years in prison and hefty fines.
Three firemen were in critical condition for burns they sustained fighting a fire near Ourense in the northwestern region of Galicia.
Spain's wildfires are being fanned by one of the longest heatwaves since records began. They have razed about 148,000ha in Spain this year, the second-biggest area since 2006.
That makes up more than a quarter of the 512,000ha burned in the EU so far this year, according to the European Commission's European Forest Fire Information System.
In Galicia, about 170 children and those supervising them spent Tuesday confined to their summer camp grounds after a wildfire burned the cables of the camp's power system, the regional government said in a statement.
The campers were rescued and moved to a water park on Wednesday.
"We were really scared, but in the midst of the fire, with a few tears, we and the monitors were also dancing. The sky was orange, but everything worked out in the end," 12-year-old Carlota told news agency Europa Press.
Spain received two Canadair water-bombing planes from France on Thursday after it asked for help from European partners to tackle the fires, Interior Minister Fernando Grande-Marlaska said in an interview on RTVE.
Nearly two dozen blazes continued to rage, fuelled by wind and extreme heat. Around 9500 people were evacuated from their towns and hundreds ordered to stay at home.
