The Treaty establishing a Constitution for Europe (TCE), commonly referred to as the European Constitution, was an unimplemented international treaty intended to create a constitution for the European Union. It was signed in 2004 by representatives of the member states of the Union but was subject to ratification by all member states, two of which subsequently rejected it in referendums. Its main aims were to replace the overlapping set of existing treaties (see Treaties of the European Union) that compose the Union's current informal constitution, to codify human rights throughout the EU and to streamline decision-making in what is now a 27-member organisation.

The TCE was signed in Rome by representatives of the member states on 29 October 2004, and was in the process of ratification by the member states when, in 2005, French (29 May) and Dutch (1 June) voters rejected the treaty in referenda. The failure of the treaty to win popular support in these two countries caused some other countries to postpone or halt their ratification procedures, and the European Council (of heads of government of the Member States) to call a "period of reflection". Had it been ratified by all Member States, the treaty would have come into force on 1 November 2006. In perspective, 18 member states ratified the text (three by referendum: Spain, Luxembourg and Romania) while 7 postponed the ratification process after the 2 rejections.

Following the period of reflection, the European Council meeting in June 2007 decided to start negotiations on a Reform Treaty as a replacement.