The European Court of Justice (ECJ), formally just the Court of Justice, (French: Cour de Justice), is the supreme court of the European Union in matters of European Union law. As a part of the Court of Justice of the European Union, it is tasked with interpreting EU law and ensuring its uniform application across all EU member states under Article 263 of the Treaty of the Functioning of the European Union (TFEU).

The Court was established in 1952, and is based in Luxembourg. It is composed of one judge per member state – currently 27 – although it normally hears cases in panels of three, five or fifteen judges. The Court has been led by president Koen Lenaerts since 2015.

The ECJ is the highest court of the European Union in matters of Union law, but not national law. It is not possible to appeal against the decisions of national courts in the ECJ, but rather national courts refer questions of EU law to the ECJ. However, it is ultimately for the national court to apply the resulting interpretation to the facts of any given case, although only courts of final appeal are bound to refer a question of EU law when one is addressed. The treaties give the ECJ the power for consistent application of EU law across the EU as a whole.

The court also acts as an administrative and constitutional court between the other EU institutions and the Member States and can annul or invalidate unlawful acts of EU institutions, bodies, offices and agencies.