The Western European Union (WEU; French: Union de l'Europe occidentale, UEO; German: Westeuropäische Union, WEU) was the international organisation and military alliance that succeeded the Western Union (WU) after the 1954 amendment of the 1948 Treaty of Brussels. The WEU implemented the Modified Brussels Treaty. During the Cold War, the Western Bloc included the WEU member states and the United States and Canada as part of the North Atlantic Treaty Organization (NATO).

At the turn of the 21st century, after the end of the Cold War, WEU tasks and institutions were gradually transferred to the European Union (EU), providing central parts of the EU's new military component, the European Common Security and Defence Policy (CSDP). This process was completed in 2009 when a solidarity clause between the member states of the European Union, which was similar (but not identical) to the WEU's mutual defence clause, entered into force with the Treaty of Lisbon. The states party to the Modified Treaty of Brussels consequently decided to terminate that treaty on 31 March 2010, with all the WEU's remaining activities to be ceased within 15 months. On 30 June 2011, the WEU was officially declared defunct.