A Croatian, right, and a EU flag fly in downtown in Zagreb, Croatia, Sunday, Jun. 30, 2013.
Croatians have celebrated across the country as it became the 28th member of the European Union.
Fireworks lit the sky as the membership became effective at midnight (2200 UTC). At Croatia's borders with other members, officials began unveiling EU signs and removing customs checkpoints.
More than 100 European dignitaries attended the event at the main square in the capital, Zagreb.
U.S. Secretary of State John Kerry congratulated Zagreb and said it was time to "honor Croatia and the many achievements of its citizens."
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 come amid 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.
Croatia Becomes Newest EU Member
1/4Fireworks illuminate the sky over the border crossing between Croatia and Slovenia during celebrations of Croatia's EU accession, in Bregana, July 1, 2013.
2/4A Croatian official unveils a new EU border sign at Bajakovo border crossing between Serbia and Croatia, July 1, 2013.
3/4A Croatian police office and a customs officer raise a European Union flag at the border crossing between Croatia and Slovenia during celebrations marking Croatia's EU accession, in Bregana, July 1, 2013.
4/4A woman makes victory sign as she holds the EU flag during the celebration of the accession of Croatia to the European Union in Zagreb, June 30, 2013.
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