Fanned by strong winds and a heatwave, the blaze has already destroyed some 600 hectares (1,500 acres) of forest and scrubland in Galicia, the regional government said in a statement.
“Everything indicates it was started intentionally,” it said.
The fire started out on Wednesday in several spots near the town of Verin near the border with Portugal, which is experiencing its worst drought in a century.
They authorities said the blaze did not currently pose a threat to “inhabited areas”.
Scientists say human-induced climate change is making extreme weather events including heatwaves and droughts more frequent and more intense. They in turn increase the risk of fires, which emit climate heating greenhouse gases.
Spain has battled 354 wildfires since the start of the year, fuelled by scorching temperatures and drought conditions.
The flames have destroyed nearly 230,000 hectares, more than in any other nation in Europe, according to the European Union’s satellite monitoring service EFFIS.
Temperatures are set to soar above 40 degrees Celsius (104 degrees Fahrenheit) on Thursday in the south and east.