Slovakia has become the 16th member of the eurozone - the second former communist country to join the grouping.
Up to 100,000 people gathered in the capital Bratislava's main square for a midnight ceremony with fireworks.
Slovak Prime Minister Robert Fico was one of the first to withdraw the new currency, taking 100 euros from a cash machine in the parliament building.
The Slovak koruna (crown) will remain in circulation alongside the euro until 16 January.
Cash machines were meant to be issuing euros from Thursday, while some banks planned to open, despite the New Year's Day holiday, to swap korunas for euros.
But the new currency was taking a while to filter through.