Firefighters backed by aeroplanes and helicopters were battling to curb flames that ravaged 1,200 hectares (2,965 acres) as they spread, the regional government said in a statement.
An alert was declared after the fire came close to some houses and residents were evacuated, an official in the Galicia region rural affairs ministry told AFP, but no one was reported hurt.
“The situation has improved since last night,” the official said. “We are not aware of the fire reaching any houses, although there were some evacuations.”
The official could not say how many people were evacuated but Spanish newspapers said the figure was at least 176.
The ministry said firefighters had controlled four other fires in the region in recent days, which burned thousands of hectares between them.
The central government said it had sent firefighting planes and personnel to the Oia fire and two others, in the Asturias region and the Tarragona area west of Barcelona.
Spain and Portugal are prone to forest fires in summer because of soaring temperatures, strong winds and dry vegetation.
Last year wildfires destroyed more than 150,000 hectares of land in Spain from January to July, after one of the driest winters on record.
This year the winter was wetter and there have been fewer summer fires so far.
And almost 31,000 hectares (77,000 acres) of Portugal’s countryside has already been destroyed by fire this year, according to its Institute for the Conservation of Nature and Forests.