The Treaty of Nice (or Nice Treaty) was signed by European leaders on 26 February 2001 and came into force on 1 February 2003. It amended the Maastricht Treaty (or the Treaty on European Union) and the Treaty of Rome (or the Treaty establishing the European Community). The Treaty of Nice reformed the institutional structure of the European Union to withstand eastward expansion, a task which was originally intended to have been done by the Amsterdam Treaty, but failed to be addressed at the time.

The entrance into force of the treaty was in doubt for a time, after its initial rejection by Irish voters in a referendum in June 2001. This referendum result was reversed in a subsequent referendum held a little over a year later.