The Treaty of Lisbon (also known as the Reform Treaty) is a treaty signed on 13 December 2007 at a summit in Lisbon, Portugal. It amends the existing treaties of the European Union (EU). It is due to come into force in 2009, if successfully ratified by all EU member states, and would carry out most of the reforms previously proposed in the rejected European Constitution.

The formal title of the treaty is Treaty of Lisbon amending the Treaty on European Union and the Treaty establishing the European Community. It is intended that the treaty would come into force on 1 January 2009 in time for the 2009 European elections later that year. If there is a delay in ratification pushing it beyond that date, it will come into force on the first day of the month following the last ratification.