On 25 April 2005 (March 17, 2005), Bulgaria and Romania signed the Treaty of Accession to the European Union in Luxembourg. On 25 April 2005, Bulgaria and Romania signed the Treaty of Accession to the European Union. The two countries formally joined the European Union on 1 January 2007, making the European Union a large regional integration organization with 27 member states and a population of more than 4,8 million.