Colour paintings and carvings on rocks, known as petroglyphs, of wildlife and geometric patterns dating back 4,000 years, have been charred and blackened in fires in Campo Lameiro and Cotobade in north western Galicia, local government spokeswoman Iria Mendez said.
It is too early to determine if some of the art, considered national treasures, have been damaged beyond repair, Ms Mendez said.
The scorching of the priceless rock art comes as Spanish officials say 20 people have been arrested on suspicion of arson as more than 100 fires rage out of control across north-west Spain and neighbouring Portugal.
Interior Minister Alfredo Perez Rubalcaba says the fires appear to have been strategically planned with evil intentions.
The arrests include a 90-year-old man and part time Spanish fire fighter.
The 31-year-old fire fighter was arrested on suspicion of starting three fires near the Galician town of Malpica in late July and investigators want to know if he was also behind other fires.
Mr Rubalcaba said he suspected a "ruthless gang" was behind some of the more than 100 fires burning in Galicia, one of Spain's most forested regions.
Authorities say 144 fires fanned by winds and drought have been counted of which only 35 are under control despite the use of unprecedented resources including more than six thousand fire fighters, 700 vehicles and 60 aircraft.