One Spaniard was among the victims and the rest appeared to be foreign nationals who ignored instructions to shelter in place, trying instead to flee by car as flames spread rapidly through a wooded area around the town of Los Gallardos in Almeria province, said Antonio Sanz, head of emergencies in the Andalusia region.
The area is a popular holiday destination and home to many foreigners, especially the French, Britons and Belgians.
Four people, who appeared to be British because the steering wheel of their car was on the right-hand side, died in one vehicle, he said on Friday.
Seven others were found dead after apparently abandoning their cars and attempting to escape on foot along a route that was not part of the evacuation plan.
"The consequences have been terrible. Everything seems to indicate that, in the case of the dead, the majority - or all of them - are foreign nationals," Sanz said.
Juan Manuel Moreno, head of Andalusia's regional government, said 23 people were missing, some probably hikers.
Rescue workers found several walking sticks at the scene.
"I think they've been caught off-guard in the woods. When there's a sudden fire ... you don't know how to get out," Moreno said.
The circumstances resemble those in neighbouring Portugal in June 2017, when a huge wildfire during a heatwave killed more than 60 people, with half of the victims burned to death in their cars.
A series of early summer heatwaves has left large parts of Spain parched and vulnerable to any spark, fuelling an early start to wildfire season.
So far in 2026, about 57,000 hectares have burned, about half the annual average for the past two decades and making up 40 per cent of all the area burned in the European Union, according to the European Forest Fire Information System.
"We usually don't see these fires until August. They're starting earlier now because the vegetation dries out sooner," Roman Garcia, a forest firefighter from Salamanca, said on state broadcaster TVE.
A record heatwave last August provoked the worst wildfire season in three decades, charring 330,000ha, an area twice the size of London.
Prime Minister Pedro Sanchez acknowledged at the time that wildfire prevention had been "clearly insufficient" and forestry management inadequate, pledging to do "whatever it takes" to ensure fires on such a scale never happened again.
Sanchez on Friday offered his condolences to the families of the victims and said he felt "enormous sadness and devastation".
As authorities sought to identify the dead and track down the missing, anxious relatives from around the world posted messages on social media and local forums.
One woman said her daughter, who was driving a red Ford Fiesta and had her dog with her, was missing.
Another person from the United States said her brother had been among a group of 10 people who tried to escape through a valley next to a stream.
Pedro Ridao, mayor of the nearby town of Antas, told TVE the fire was believed to have started when a power cable came loose and fell on a patch of dry scrubland on Thursday.
However, a spokesperson for utility company Endesa ruled that out after technicians found that the cable carried no voltage.
The death toll already surpasses that of 2005, when 11 firefighters were killed in a blaze in the central province of Guadalajara that was sparked by a barbecue.
