The Treaty of Brussels, also referred to as the Brussels Pact, was the founding treaty of the Western Union (WU) between 1948 and 1954, when it was amended as the Modified Brussels Treaty (MTB) and served as the founding treaty of the Western European Union (WEU) until its termination in 2010. The treaty provided for the organisation of military, economic, social and cultural cooperation among member states as well as a mutual defence clause.

The treaty was signed on 17 March 1948 by Belgium, France, Luxembourg, the Netherlands and the United Kingdom, the members of the Western Union, as an expansion to the Treaty of Dunkirk, which had been signed between Britain and France the previous year to guard against possible German or Soviet aggression after the end of World War II.

The need to back up the commitments of the North Atlantic Treaty with appropriate political and military structures led to the creation of the North Atlantic Treaty Organization (NATO). In December 1950, the parties to the Treaty of Brussels decided to transfer the headquarters, personnel and plans of the Western Union Defence Organisation (WUDO) to NATO, whose Supreme Headquarters Allied Powers Europe (SHAPE) took over responsibility for the defence of Western Europe.

The establishment of NATO, along with the signing of a succession of treaties establishing the Organisation for European Economic Cooperation (April 1948), the Council of Europe (May 1949) and the European Coal and Steel Community (April 1951), left the Treaty of Brussels and its Western Union devoid of authority.

The treaty was amended at the 1954 Paris Conference as a result of the failure of the Treaty establishing the European Defence Community to gain French ratification: The General Treaty (German: Deutschlandvertrag) of 1952 formally named the EDC as a prerequisite of the end of Allied occupation of Germany, and there was a desire to include Germany in the Western defence architecture. The Modified Brussels Treaty (MBT) transformed the Western Union into the Western European Union (WEU), at which point Italy and Germany were admitted. Although the WEU that was established by the Modified Brussels Treaty was significantly less powerful and ambitious than the original Western Union, German membership of the WEU was considered sufficient for the occupation of the country to end in accordance with the General Treaty.

When the European Union (EU) gained its own mutual defence clause upon the entry into force of the Treaty of Lisbon in 2009, the members of the WEU, which were also EU member states, regarded the WEU as redundant. Consequently, the Modified Treaty of Brussels was terminated on 31 March 2010, followed by the closure of WEU bodies on 30 June 2011.