The Treaty of Accession 2005 is an agreement between the member states of European Union and Bulgaria and Romania. It entered into force on 1 January 2007. The Treaty arranged accession of Bulgaria and Romania to the EU and amended earlier Treaties of the European Union. As such it is an integral part of the constitutional basis of European Union.

Full name of the Treaty is: Treaty between the Kingdom of Belgium, the Czech Republic, the Kingdom of Denmark, the Federal Republic of Germany, the Republic of Estonia, the Hellenic Republic, the Kingdom of Spain, the French Republic, Ireland, the Italian Republic, the Republic of Cyprus, the Republic of Latvia, the Republic of Lithuania, the Grand Duchy of Luxembourg, the Republic of Hungary, the Republic of Malta, the Kingdom of the Netherlands, the Republic of Austria, the Republic of Poland, the Portuguese Republic, the Republic of Slovenia, the Slovak Republic, the Republic of Finland, the Kingdom of Sweden, the United Kingdom of Great Britain and Northern Ireland (Member States of the European Union) and the Republic of Bulgaria and Romania, concerning the accession of the Republic of Bulgaria and Romania to the European Union.