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.
Europol, which supported the investigation, said the firearms were sold in Spain, France and Belgium.
It said some of the weapons were deactivated, but did not comply with established standards.
Criminals acquired the arsenal largely through auctions and other legal channels before reactivating it.
The gang had been using a sports shop as a front for its distribution centre - which in reality sold firearms, weapon components and ammunition.
Police said the weapons would have had an easy journey onto the black market, and into the hands of terrorists or organised crime groups.
Europol said firearms traffickers exploit legal loopholes and legislative differences between EU countries to divert guns from legal suppliers.
Reactivating deactivated weapons is one of Europe's main sources of illegal guns.
The agency said it had seen a significant increase in the number being supplied to criminals since 2014.