Commisssion spokeswoman Pia Ahrenkilde Hansen told a news briefing in Brussels that Croatia's accession was proof that the goal of EU membership has a stabilizing effect on prospective candidates.
"[Croatia's accession] provides fresh evidence of the transformative power of the European Union: torn by conflict only two decades ago, Croatia is now a stable democracy capable of taking on the obligations of EU membership and of adhering to EU standards," Ahrenkilde Hansen said.
Croatia joins the EU more than two decades after its bloody splitting from former Yugoslavia and after six years of tough membership negotiations.
"Dear friends, welcome to the European Union," EU Commission President Jose Manuel Barroso had told Croatians and VIP guests in both English and Croatian at the overnight ceremony in downtown Zagreb.
Croatian Prime Mininister Zoran Milanovic told attendees his country was ready for its new role.
"Whatever anyone is saying about us, you should know we are a country of rational people who are realistic about themselves and their country and the role of their country in Europe and the world," Milanovic said. "Croatia knows it is not going away tonight from where it is, we are staying where we are but with the new responsibility, the responsibility to help the countries in the region to achieve the criteria for the EU as soon as possible."
The country gains admission to the bloc with the EU stuck in recession. Many countries are struggling to stimulate growth amid spending cuts and rising taxes to deal with a debt crisis.
Croatia is also suffering slow economic growth, and unemployment is about 20 percent in the Balkan country of more than 4 million. It joins the EU as the bloc's third-poorest country.
Milanovic had dismissed comparisons of Croatia's EU entry to boarding a sinking ship. European Parliament President Martin Schulz welcomed Croatia but said membership offers "no magic solution to the crisis."
Croatian President Ivo Josipovic pledged to help other Balkan neighbors move closer to the bloc and to help bring lasting stability to the region.
Josipovic spoke after a meeting with the presidents of Albania, Bosnia, Kosovo, Macedonia, Montenegro, Slovenia, and Serbia.
"During our talks today, we discussed three important issues, the first of them being further enlargement and the support of Slovenia and Croatia to the policy of further EU enlargement," Josipovic said. "The second issue was many opportunities for common projects, especially in infrastructure. And the third one -- our informal group would like to help in solving some open issues that are still troubling the region."
Josipovic said in his celebratory speech in Zagreb overnight that Croatia will not "let the cloud of the economic crisis overshadow our vision and optimism."
While a full member, Croatia is not yet joining the free-movement Schengen space or the 17-member common currency eurozone.
Overnight, customs posts were removed from other EU neighbors, Slovenia and Hungary, while EU signs and flags were put on its borders with non-EU states Bosnia, Serbia, and Montenegro.
Croatian police chief Vlado Dominic said at a separate ceremony held at midnight on the border with Serbia that Schengen membership is now Croatia's top priority.
"Until yesterday the Croatian policeman here was guarding around 4.5 million citizens of Croatia," Dominic said, "but as of today, he is guarding 500 million people of the European Union. Now the Croatian police and the Croatian government have the goal to enter into the Schengen zone. I hope we will be its members in two years at the earliest."
Montenegro could become the next ex-Yugoslav republic to join the EU.