Three people died in Spain's Galicia region - two of them women found inside a burnt-out auto, the third a man in his 70s, killed as he tried to save his farm animals.
"They are absolutely intentional fires, premeditated, caused by people who knew what they are doing", said Alberto Nunez Feijoo, the head of the Galicia government.
The 36 deaths, which were confirmed by Portugal's national civil protection agency and included a one-month-old baby, come four months after 64 people were killed in the deadliest fire in the country's history in June.
The fatalities in Portugal occurred in densely forested parts of central and northern part of the country after blazes broke out in "exceptional" weather circumstances, a Civil Protection Agency spokeswoman said. On Monday, the "situation remained very worrying", Feijoo said, adding that firefighters along with soldiers and locals were battling the flames.
'We went through absolute hell, it was frightful. "There was fire everywhere", a resident of the town of Penacova, near Lousa. told RTP television.
Flames ripped across countryside left tinder-dry by an unusually hot summer and early autumn, fanned by strong winds as remnants of ex-Hurricane Ophelia brushed the Iberian coast.
Government minister Jorge Gomes said that most fires had a criminal cause.
Spain's Prime Minister, Mariano Rajoy, has travelled to the northwestern region of Galicia, where thousands of people have been evacuated.
Spanish TV pictures showed people trying to fight fires with buckets of water.
In Vigo, the fires destroyed several industrial warehouses and forced the evacuation of the PSA/Citroen auto factory, a commercial center and a residence for university students.
Officials in both countries said they expected that rain and cooler weather forecast for later on Monday would help put out the fires.
Five wildfires were raging near Vigo, Galicia's biggest city, forcing authorities to close schools, roads and factories - with people taking shelter in sports centres and hotels. The fire prompted the evacuation of hundreds of people.
Two women died on Sunday after being engulfed in flames trapped in their van near Nigran, outside Vigo, and an elderly man died in an animal shed near his house in Carballeda de Avia.
Meteorologists said Ophelia was the most powerful hurricane recorded so far east in the Atlantic and the first since 1939 to travel so far north.