In international law, a condominium (plural either condominia, as in Latin, or condominiums) is a political territory (state or border area) in or over which multiple sovereign powers formally agree to share equal dominium (in the sense of sovereignty) and exercise their rights jointly, without dividing it into "national" zones.
Although a condominium has always been recognized as a theoretical possibility, condominia have been rare in practice. A major problem, and the reason so few have existed, is the difficulty of ensuring co-operation between the sovereign powers; once the understanding fails, the status is likely to become untenable.
The word is recorded in English since c. 1714, from Modern Latin, apparently coined in Germany c. 1700 from Latin com- "together" + dominium "right of ownership" (compare domain). A condominium of three sovereign powers is sometimes called a tripartite condominium or tridominium.
- The International Space Station is a de facto space condominium, via a program of complex set of legal, political and financial agreements between all parties.
- Antarctica is a de facto continental condominium, governed by parties to the Antarctic Treaty that have consulting status.
- The Moselle and its tributaries, the Sauer and the Our, constitute a condominium between Luxembourg and Germany, which also includes bridges, about 15 river islands of varying size, and the tip of one island, Staustufe Apach, near Schengen (the rest of the island is in France). The condominium was established by treaty in 1816.
- Pheasant Island (also known as Conference Island, Konpantzia in Basque, Île de la Conférence in French or Isla de los Faisanes in Spanish) in the Bidassoa is a condominium between France and Spain. It was established by the Treaty of the Pyrenees in 1659. It is formally controlled by Spain between 1 February and 31 July each year and by France for the following six months. The island has no permanent population and has been eroded significantly by the river.
- El Salvador, Honduras and Nicaragua exercise a tridominium over parts of the Gulf of Fonseca and of the territorial sea outside its mouth.
- Austria and Germany consider themselves, together with Switzerland, to hold a tripartite condominium (albeit on different grounds) over the main part of Lake Constance (without its islands). On the other hand, Switzerland holds the view that the border runs through the middle of the lake. Hence no international treaty establishes where the borders of Switzerland, Germany, and Austria in or around Lake Constance lie.
- The part of the Paraná River between the Salto Grande de Sete Quedas and the mouth of the Iguassu River is shared in condominium by Brazil and Paraguay.
- Colombia and Jamaica share a maritime condominium (called a "Joint Regime Area") by mutual agreement as an alternative to delimiting their sea boundary. The outer portion of the EEZ of each country otherwise would overlap in this area. Unlike other "joint development zones", this condominium appears not to have been purposed simply as a way to divide oil, fisheries or other resources.
- In Bosnia and Herzegovina, the Brčko District forms a condominium between the Federation of Bosnia and Herzegovina and Republika Srpska.
Under French law, Andorra was once considered to be a French–Spanish condominium, although it is more commonly classed as a co-principality, since it is itself a sovereign state, not a possession of one or more foreign powers. However, the position of Head of State is shared ex officio by two foreigners, one of whom is the President of France, currently Emmanuel Macron, and the other the Bishop of Urgell in Spain, Joan Enric Vives Sicília.
- In 688 the Byzantine Emperor Justinian II and the Arab Caliph Abd al-Malik ibn Marwan reached an unprecedented agreement to establish a condominium (the concept did not yet exist) over Cyprus, with the collected taxes from the island being equally divided between the two parties. The arrangement lasted for some 300 years, even though in the same time there was nearly constant warfare between the two parties on the mainland.
- Fellesdistrikt (Common District) (Norwegian: Fellesdistrikt, Russian: Общий район) was a Russo-Norwegian condominium from 1684 until 1826.
- Anglo-Egyptian Sudan was legally a Egyptian-British condominium from 1899 until 1956, although in reality Egypt played no role in its government other than providing some administrators in the country: all political decisions were made by the UK and all Governors-General of Anglo-Egyptian Sudan were British. The system was resented by Egyptian and Sudanese nationalists, and would later be disavowed by the Egyptian Government, it persisted due to the United Kingdom's effective control over Egypt itself, which began from 1882 and continued until at least 1936.
- The Condominium of Bosnia and Herzegovina was jointly ruled by Cisleithanian Austria and Hungary between 1908 and 1918, while both countries were parts of the Austro-Hungarian Empire.
- An islet in the border river Brömsebäck was considered to belong to neither (or both) Denmark and Sweden.
- The Independent State of Croatia during World War II from 1941 to 1943 was a condominium of Nazi Germany and Italy until 1943, when Italy split between the allied-leaning Kingdom of Italy under King Victor Emmanuel III and axis-leaning Italian Social Republic under Benito Mussolini.
- Canton and Enderbury Islands were a British–American condominium from 1939 until 1979 when they became part of Kiribati
- Couto Misto was shared until 1864 between Spain and Portugal, even though in its final decades of existence it was de facto independent.
- Countship of Friesland (West Frisia), since 1165 under Imperial administration, was from 1165 to 1493 a joint condominium of the Count of Holland and the Prince-bishop of Utrecht, then again until 25 October 1555 under Imperial administration.
- Egypt from 1876–1882 was under France and the United Kingdom.
- The city of Erfurt from 12th century until the Thirty Years' War was shared between the Archbishopric of Mainz and the Counts of Gleichen, the latter replaced by the city council in 1289 (Concordata Gebhardi), the Landgrave of Thuringia in 1327 and the House of Wettin in 1483 (Treaty of Weimar).
- A small area (Hadf and surroundings) on the Arabian Peninsula, a part of Oman, at one time was jointly ruled with the Emirati member state of Ajman. The agreement defining the Hadf zone was signed in Salalah on 26 April 1960 by Sultan Said bin Taimur and in Ajman on 30 April 1960 by Shaikh Rashid bin Humaid Al Nuaimi, ruler of Ajman. This provided for some joint supervision in the zone by the ruler of Ajman and the shaikhs under the rule of Muscat. It allowed the Ajman ruler to continue collecting zakat (Islamic tax). The ruler of Ajman was, however, not to interfere in the affairs of the local people, the Bani Ka'ab (a branch of the Banu Kaab), which were the sole responsibility of shaikhs who were under Muscat rule. The agreement was later terminated.
- The Free City of Kraków was a protectorate of Prussia, Austria and Russia from 1815 until 1846, when it was annexed by Austria
- Maastricht was a condominium for five centuries until 1794. It was shared between the Prince-Bishopric of Liège and the Duchy of Brabant, the latter replaced by the Dutch Republic in 1632
- Nauru was a tripartite condominium mandate territory administered by Australia, New Zealand and United Kingdom from 1923 to 1942 and again in 1947 as a trust territory until independence in 1968
- The village of Nennig was a condominium of the Trier bishopric, Lorraine (the Kingdom of France from 1766) and Duchy of Luxembourg until its annexation by Revolutionary France in 1794
- Neutral Moresnet was shared from 1816 until 1919 between the Netherlands (later Belgium) and Prussia
- New Hebrides formed a French–British condominium in 1906 until independence in 1980 as a republic, now called Vanuatu
- Northern Dobruja was shared by the Central powers (German-Austrian-Bulgarian) during World War I.
- The Oregon Country was an Anglo-American condominium from 1818 until 1846
- Samoan Islands from 1889 to 1899 were a rare tripartite condominium under joint protectorate of Germany, Britain and the United States.
- Schleswig, Holstein and Lauenburg were an Austrian-Prussian condominium. The two German powers acceded it following the 1864 Second Schleswig War. According to the Gastein Convention in the next year, Lauenburg left the condominium (to Prussia), Austria governed Holstein and Prussia Schleswig. In 1866, after the Austro-Prussian War, Austria passed over its remaining rights to Prussia. Prussia later created the Province of Schleswig-Holstein.
- The County of Sponheim in the Holy Roman Empire was ruled since the 15th century by the Margraves of Baden, the Counts Palatine of the Rhine and the Counts of Veldenz, later Palatinate-Simmern, Palatinate-Zweibrücken and Palatinate-Birkenfeld as heirs of Veldenz.
- The Spanish Netherlands became an Anglo-Dutch condominium in 1706 during the War of the Spanish Succession, until the peace treaties of Utrecht and Rastatt in 1713/14 ending the war.
- Togoland, formerly a German protectorate, was an Anglo-French condominium, from when the United Kingdom and France occupied it on 26 August 1914 until its partition on 27 December 1916 into French and British zones. The divided Togoland became two separate League of Nations mandates on 20 July 1922: British Togoland, which joined Gold Coast (present-day Ghana) in 1956, and French Togoland, which is now the nation of Togo.
- Zaporozhian Sich, a brief Russo-Polish condominium, was established in 1667 by the Treaty of Andrusovo.
- Tangier International Zone, an international zone nominally under Moroccan sovereignty but jointly administered by several European powers.
- The term is sometimes even applied to a similar arrangement between members of a Monarch's countries in (personal or formal) union, as was the case for the district of Fiume (Rijeka), shared between Hungary and Croatia within the Habsburg Empire since 1868.
- Between 1913 and 1920 Spitsbergen was a neutral condominium. The Spitsbergen Treaty of 9 February 1920 recognises the full and absolute sovereignty of Norway over all the arctic archipelago of Svalbard. The exercise of sovereignty is, however, subject to certain stipulations, and not all Norwegian law applies. Originally limited to nine signatory nations, over 40 are now signatories of the treaty. Citizens of any of the signatory countries may settle in the archipelago. Currently, only Norway and Russia make use of this right.
- In 1992, South Africa and Namibia established a Joint Administrative Authority in the enclave of Walvis Bay, prior to its cession to Namibia in 1994.
- In 2001, the British government held discussions with Spain with a view to putting a proposal for joint sovereignty to the people of Gibraltar. This initiative was pre-emptively rejected by Gibraltarians in the 2002 referendum.
- In 2012, the Canadian and Danish governments were close to an agreement to declare Hans Island a condominium, after decades in dispute. Another considered alternative was to divide the island in half. Negotiations continued.
- Some proposed two-state solutions to the Israeli–Palestinian conflict involve a State of Israel and State of Palestine sharing sovereignty over part or all of Jerusalem.
- In the talks between the UK and the People's Republic of China in 1983–84 over the transfer of sovereignty over Hong Kong, one of the British proposals was to transfer the sovereignty of Hong Kong and its dependencies to the People's Republic of China while the UK would retain the rights of administration of the territory. The proposal was rejected and negotiations ended with the UK agreeing to relinquish all rights over Hong Kong to China in 1997.
- In one proposed case of the Partition of Belgium, Brussels would become a condominium of Flanders and Wallonia.
- In 1984, the New Ireland Forum suggested a proposal for joint UK/Irish authority in Northern Ireland to try to bring an end to the Troubles conflict. This idea was dismissed by the British government. The Good Friday Agreement of 1998 led to the establishment of a devolved power-sharing executive in Northern Ireland led jointly by the First and Deputy First Minister, positions nominated separately by the largest parties in each of the two largest community designations in the assembly. The proposal of joint authority by the UK and Irish governments over Northern Ireland was raised again in 2017 after the collapse of the power-sharing executive.