It includes over 10,000 assault rifles, machine guns, pistols, revolvers, and 400 shells and grenades.
The guns and ammunition were seized in January during an operation against firearms trafficking.
Investigators also found an illegal workshop with machinery to manipulate and reactivate weapons, near Bilbao. Five people were arrested.
Cash amounting to 80,000 euros (£70,000 / $85,000) was seized.
The operation involved counter-terror police from Madrid, Bilbao, Valencia and Gerona.