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 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.
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.
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 688 the Byzantine EmperorJustinian II and the Arab CaliphAbd 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.
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.
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.
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.
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 a 2019 essay, former Australian Prime Minister Kevin Rudd suggested that Australia should consider a "formal constitutional condominium" with Tuvalu, Kiribati, and Nauru as a solution to the impacts of climate change. Rudd outlined that the people of these countries would be offered citizenship and relocation to Australian territory in exchange for Australian ownership of their territorial seas, Exclusive Economic Zones, and fisheries.
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.
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 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.
^Jozo Tomasevich. "The Chetniks". War and Revolution in Yugoslavia. Stanford University Press, 1975. Pp. 103. "The condominium in Croatia was the most important example of Italo-German collaboration in controlling and despoiling an occupied area [...]".
^Stephen R. Graubard, (ed.).Exit from Communism. Transaction Publishers, 1993. Pp. 153–154. "After the Axis attack on Yugoslavia in 1941, Mussolini and Hitler installed the Ustašas in power in Zagreb, making them the nucleus of a dependent regime of the newly created Independent State of Croatia, an Italo-German condominium predicated on the abolition of Yugoslavia." 
^Günay Göksu Özdoğan, Kemâli Saybaşılı. Balkans: a mirror of the new international order. Marmara Üniversitesi. Dept. of International Relations, 1995. Pp. 143. "Croatia (with Bosnia-Hercegovina) formally became a new Axis ally – the
Independent State of Croatia (NDH). This was in fact, Italo-German condominium, [...]".
^John R. Lampe (ed.), Mark Mazower (ed.). Ideologies and National Identities: The Case of Twentieth-Century Southeastern Europe. Central European University Press, 2003. Pp. 103. "[...] the Independent State of Croatia (hereafter NDH, Nezavisna Drzava
Hrvatska), in reality an Italo-German condominium[...]"