"It is a unique collection and there are very few similar cases," Ana Navarro, head of Seville's Archeology Museum which is looking after the find, told a news conference.
Dating back to the late third and early fourth centuries, the bronze coins were found Wednesday inside 19 Roman amphoras, a type of jar, in the town of Tomares near Seville.
Navarro declined to give a precise estimate for the value of the haul, saying only that the coins were worth "certainly several million euros".
The coins are stamped with the inscriptions of emperors Maximian and Constantine, and they appeared not to have been in circulation as they show little evidence of wear and tear.
It is thought they were intended pay the army or civil servants.
"The majority were newly minted and some of them probably were bathed in silver, not just bronze," said Navarro.
"I could not give you an economic value, because the value they really have is historical and you can't calculate that."
Local officials have suspended the work on the water pipes and plan to carry out an archaeological excavation on the site.
The Romans conquered the Iberian Peninsula in 218 BC, ruling until the early 5th century when they were ousted by the Visigoths.