The EU chief said without the single currency the continent would face “tremendous conflict” between states including “military war”.
And in a speech likely to raise hackles amongst struggling Mediterranean countries he claimed all Europe would be “much more impoverished” without it.
Mr Juncker made the remarks during an address to students at the University of Luxembourg, in which he also called for closer ties with Russia.
His comments will anger those voters, particularly in countries like Italy and Greece, who feel the euro has penalised them at the expense of Germany.
The euro was introduced in 2000 and has endured a tricky 17-year life, largely down to it operating within an incomplete fiscal framework.
Senior EU politicians like Emmanuel Macron and Mr Juncker are now calling for the eurozone to be “completed”, but countries like Germany are resistant to any idea of debt pooling.
In his speech today, the EU chief said: “Who would’ve thought that that Single Market would actually develop to the stage we’re at today where all of the trade barriers and others have actually been eroded, where there are no longer such obstacles between member states.
“Who would have thought that we’d have been able to bring together 19 different currencies to create one single European currency, a currency which today protects us.”
In an eyebrow raising intervention, he then added: “And if we didn’t have that, we would have tremendous conflict and consequences including not just military war, but also economic difficulties.
“We would be much more vulnerable if we didn’t have our single currency we’d be much, much more impoverished too without our single currency.
“We’ve seen the crises in Europe that affect many of the things we thought we’d achieved in Europe, many of the things we’ve created are under threat leading some people to the exit door.”
In his speech Mr Juncker also admitted that “inequalities within our societies seem to be increasing” and that this was the main “risk” lies in terms of further EU integration.
And he also made some contentious remarks about future relations with Russia which are likely to unnerve some of the Eastern European states, especially those in the Baltic.
The EU boss said that Europe as a continent was not big or powerful enough to turn a blind eye to Moscow’s actions and that it would need to “build bridges” with Valdimir Putin in the future.
Brussels is currently pursuing a sanctions regime against the Kremlin over its illegal invasion of Ukraine back in 2014, but some member states have been calling for this to be loosened.
Mr Juncker said: “With our small surface area do we really think that we can that we can actually think of the future of Europe without looking at what the greater Russian territory is doing?
“They’re our neighbour and with that neighbour we’ve got to work don’t forget there are so many conflicts still there and we’re going to have to build bridges with Russia in the future.”