With signatures put on the document by 27 EU member-states' leaders, Croatia's President Ivo Josipovic and Prime Minister Jadranka Kosor, Croatia changed its status of a candidate country to that of an acceding country.
The treaty was signed at 1033 hours by Croatia's President Ivo Josipovic and Prime Minister Jadranka Kosor, which was welcomed by a round of strong applause.
The first country to sign the document was Belgium and the last the United Kingdom.
The ceremony started at 1025 hrs and lasted exactly 20 minutes.
The treaty stipulates that Croatia will join the EU on 1 July 2013. Before that, an EU entry referendum will be held in Croatia and the treaty will be ratified by all EU member states.
During the ratification process, Croatia will be informed and consulted about new laws to be adopted by the EU, and it will also be able to contribute to the elaboration of those laws.