Tens of thousands celebrated across Croatia as it became the 28th member of the European Union.
Fireworks lit the sky as the membership became effective at midnight (22:00 GMT). At Croatia's borders with other members, officials began unveiling EU sign and removing customs checkpoints.
More than 100 European dignitaries attended the event at the Zagreb main square.
The country of 4.4 million people is only the second of the seven states carved from federal Yugoslavia to enter the EU, following Slovenia in 2004. It is the bloc's first addition since Bulgaria and Romania joined in 2007.
It represents a milestone in Croatia's recovery from a 1991-95 war to secure independence in which some 20,000 people died.
The celebrations, however, were overshadowed by economic worries. Croatia's tourism-oriented economy has been either in recession or stagnant for the past four years.
EU figures show that the country, where unemployment stands at around 20 percent, is now among the bloc's poorest.