Politics of the International Space Station have been affected by superpower rivalries, international treaties and funding arrangements. The Cold War was an early factor, overtaken in recent years by United States distrust of China. The station has an international crew, with the use of their time, and that of equipment on the station, being governed by treaties between participant nations.
There is no fixed percentage of ownership for the whole space station. Rather, Article 5 of the IGA sets forth that each partner shall retain jurisdiction and control over the elements it registers and over personnel in or on the Space Station who are its nationals. Therefore, for each ISS module only one partner retains sole ownership. Still, the agreements to use the space station facilities are more complex.
In 1978 a milestone was reached in co-operation between the United States and the Soviet Union in space with the Apollo-Soyuz Test Project. The project occurred during a period of détente between the two superpowers, and led in July 1975 to Soyuz 19 docking with an Apollo spacecraft.
From 1978 to 1987, the USSR's Interkosmos program included allied Warsaw Pact countries, and countries which were not Soviet allies, such as India, Syria and France, in crewed and uncrewed missions to Space stations Salyut 6 and 7. In 1986, the USSR extended its co-operation to a dozen countries in the Mir program. From 1994 to 1998, NASA Space Shuttles and crew visited Mir in the Shuttle–Mir program.
In 1998, assembly of the space station began. On 28 January 1998, the Space Station Intergovernmental Agreement (IGA) was signed. This governs ownership of modules, station usage by participant nations, and responsibilities for station resupply. The signatories were the United States of America, Russia, Japan, Canada and eleven member states of the European Space Agency (Belgium, Denmark, France, Germany, Italy, The Netherlands, Norway, Spain, Sweden, Switzerland, and the United Kingdom). With the exception of the United Kingdom, all of the signatories went on to contribute to the Space Station project. A second layer of agreements was then achieved, a memoranda of understanding between NASA and ESA, CSA, RKA and JAXA. These agreements are then further split, such as for the contractual obligations between nations, and trading of partners' rights and obligations. Use of the Russian Orbital Segment is also negotiated at this level.
In 2010, the ESA announced that European countries which were not already part of the program would be allowed access to the station in a three-year trial period.
In March 2012, a meeting in Quebec City between the leaders of the space agencies of Canada, Japan, Russia, the United States and involved European nations resulted in a renewed pledge to maintain the space station until at least 2020. NASA reports to be still committed to the principles of the mission but also to use the station in new ways, which were not elaborated. CSA President Steve MacLean stated his belief that the station's Canadarm will continue to function properly until 2028, alluding to Canada's likely extension of its involvement beyond 2020.
Brazil joined the ISS as a partner of the United States and this included a contract with NASA to supply hardware to the Space Station. In return, NASA would provide Brazil with access to NASA ISS facilities on-orbit, as well as a flight opportunity for one Brazilian astronaut during the course of the ISS program. However, due to cost issues, the subcontractor Embraer was unable to provide the promised ExPrESS pallet, and Brazil left the program in 2007. Regardless, the first Brazilian astronaut, Marcos Pontes, was sent to ISS in April 2006 for a short stay during the Expedition 13 where he realized the Missão Centenário. This was Brazil's first space traveler and he returned to Earth safely. Pontes trained on the Space Shuttle and Soyuz, but ended up going up with the Russians, although he did work at the U.S. Johnson Space Center after returning to Earth.
China is not an ISS partner, and no Chinese nationals have been aboard. China has its own contemporary human space program, Project 921, and has carried out co-operation and exchanges with countries such as Russia and Germany in human and robotic space projects. China launched its first experimental space station, Tiangong 1, in September 2011, and has officially initiated the permanently crewed Chinese space station project.
In 2007, Chinese vice-minister of science and technology Li Xueyong said that China would like to participate in the ISS. In 2010, ESA Director-General Jean-Jacques Dordain stated his agency was ready to propose to the other 4 partners that China be invited to join the partnership, but that this needs to be a collective decision by all the current partners. While ESA is open to China's inclusion, the US is against it. US concerns over the transfer of technology that could be used for military purposes echo similar concerns over Russia's participation prior to its membership. Concerns over Russian involvement were overcome and NASA became solely dependent upon Russian crew capsules when its shuttles were grounded after the Columbia accident in 2003, and again after its retirement in 2011.
The Chinese government has voiced a belief that international exchanges and co-operation in the field of aerospace engineering should be intensified on the basis of mutual benefit, peaceful use and common development. China's crewed Shenzhou spacecraft use an APAS docking system, developed after a 1994–1995 deal for the transfer of Russian Soyuz spacecraft technology. Included in the agreement was training, provision of Soyuz capsules, life support systems, docking systems, and space suits. American observers comment that Shenzhou spacecraft could dock at the ISS if it became politically feasible, whilst Chinese engineers say work would still be required on the rendezvous system. Shenzhou 7 passed within about 50 kilometres of the ISS.
American co-operation with China in space is limited, though efforts have been made by both sides to improve relations, but in 2011 new American legislation further strengthened legal barriers to co-operation, preventing NASA co-operation with China or Chinese owned companies, even the expenditure of funds used to host Chinese visitors at NASA facilities, unless specifically authorized by new laws, at the same time China, Europe and Russia have a co-operative relationship in several space exploration projects. Between 2007 and 2011, the space agencies of Europe, Russia and China carried out the ground-based preparations in the Mars500 project, which complement the ISS-based preparations for a human mission to Mars.
On 28 April 2021 China launched the first part of a 11 series module space station named Tiangong Space Station. The Tianhe module was launched from the Wenchang Space Launch Centre on a Long March-5B rocket, which contains only living quarters for crew members. The entire space station when constructed will require 10 additional launches between years 2021 through to 2022.
The heads of both the South Korean and Indian space research organisation ISRO announced at the first plenary session of the 2009 International Astronautical Congress that their nations wished to join the ISS program, with talks due to begin in 2010. 
Italy has a contract with NASA to provide services to the station, and also takes part in the program directly via its membership in ESA.