The raiders -- believed to be environmental activists – moved in on farms in three towns in the Galicia region, which has about 80 mink farms.
The operation was so well organised that those responsible propped boards on the walls to help animals scale them and placed fish outside the walls as bait for them to keep going, said Maria Dolores Sendon, a police official in the town of Muros.
"This was not a prank," she said in a telephone interview. "It was very well planned."
There has been no claim of responsibility and no arrests have been made, said Jose Benito Reza, a conservation official with the Galician regional government.
An estimated 5,000 of the prized animals were released from their cages at a farm in Muros, and about 2,000 of them made it outside the walled farm compound, Sendon said.
The biggest raid was at a farm in the town of Oza dos Rios, where about 11,000 of the animals were let out of their cages, and about half made it outside the walls, said farm owner Charo Carrillo.
She gave no figure for financial losses but told the national news agency Efe the raid meant "20 years of work to create a high-quality product have been ruined".
Ms Carrillo said most of those that got away would probably starve to death in a matter of days because they were raised in captivity and did not know how to hunt or fish.
Spain raises about 400,000 mink a year, and 80 per cent are bred in Galicia, according to a Barcelona-based animal rights group called the Fundacion Altarriba.
Ms Sendon said other mink farms had been hit by eco-raiders, but these were the first such incidents in these three towns in the coastal province of La Coruna.
Last year, animal rights activists freed 30,000 mink from a farm near the regional capital, Santiago de Compostela, and painted graffiti on the wall to claim responsibility.
Benito Reza said the people who freed the latest batch "did them no favour whatsoever" because they cannot survive in the wild, and the mink might attack other animals and birds.
He also warned people against trying to catch them, saying the minks could bite.