The European Communities Act 1972, also known as the ECA 1972, was an act of the Parliament of the United Kingdom which made legal provision for the accession of the United Kingdom as a member state to the three European Communities (EC) – the European Economic Community, European Atomic Energy Community (Euratom), and the European Coal and Steel Community ; the EEC and ECSC subsequently became the European Union.

The accession of the United Kingdom to the European Communities (EC) – the collective term for the European Coal and Steel Community (ECSC), the European Economic Community (EEC) and the European Atomic Energy Community (EAEC) – took effect on 1 January 1973. This followed ratification of the Accession treaty which was signed in Brussels on 22 January 1972 by the Conservative prime minister Edward Heath, who had pursued the UK's application to the EEC since the late 1950s. The ECSC and EEC would later be integrated into the European Union under the Maastricht and Lisbon treaties in the early 1990s and mid-2000s.