The Merger Treaty, also known as the Treaty of Brussels, was a European treaty which unified the executive institutions of the European Coal and Steel Community (ECSC), European Atomic Energy Community (Euratom) and the European Economic Community (EEC). The treaty was signed in Brussels on 8 April 1965 and came into force on 1 July 1967. It set out that the Commission of the European Communities should replace the High Authority of the ECSC, the Commission of the EEC and the Commission of Euratom, and that the Council of the European Communities should replace the Special Council of Ministers of the ECSC, the Council of the EEC and the Council of Euratom. Although each Community remained legally independent, they shared common institutions (prior to this treaty, they already shared a Parliamentary Assembly and Court of Justice) and were together known as the European Communities. This treaty is regarded by some as the real beginning of the modern European Union.

This treaty was abrogated by the Amsterdam Treaty signed in 1997:

Without prejudice to the paragraphs following hereinafter, which have as their purpose to retain the essential elements of their provisions, the Convention of 25 March 1957 on certain institutions common to the European Communities and the Treaty of 8 April 1965 establishing a Single Council and a Single Commission of the European Communities, but with the exception of the Protocol referred to in paragraph 5, shall be repealed.