The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status of the natural world and the measures needed to safeguard it. It is involved in data gathering and analysis, research, field projects, advocacy, and education. IUCN's mission is to "influence, encourage and assist societies throughout the world to conserve nature and to ensure that any use of natural resources is equitable and ecologically sustainable".

Over the past decades, IUCN has widened its focus beyond conservation ecology and now incorporates issues related to sustainable development in its projects. IUCN does not itself aim to mobilize the public in support of nature conservation. It tries to influence the actions of governments, business and other stakeholders by providing information and advice and through building partnerships. The organization is best known to the wider public for compiling and publishing the IUCN Red List of Threatened Species, which assesses the conservation status of species worldwide.

IUCN has a membership of over 1,400 governmental and non-governmental organizations from over 170 countries. Some 16,000 scientists and experts participate in the work of IUCN commissions on a voluntary basis. It employs over 900 full-time staff in more than 50 countries. Its headquarters are in Gland, Switzerland. Every four years, IUCN convenes for the IUCN World Conservation Congress where IUCN Members set the global conservation agenda by voting on recommendations and guide the secretariat's work by passing resolutions and the IUCN Programme.

IUCN has observer and consultative status at the United Nations, and plays a role in the implementation of several international conventions on nature conservation and biodiversity. It was involved in establishing the World Wide Fund for Nature and the World Conservation Monitoring Centre. In the past, IUCN has been criticized for placing the interests of nature over those of indigenous peoples. In recent years, its closer relations with the business sector have caused controversy.

IUCN was established in 1948. It was initially called the International Union for the Protection of Nature (1948–1956) and has also been formerly known as the World Conservation Union (1990–2008).

IUCN was established on 5 October 1948, in Fontainebleau, France, when representatives of governments and conservation organizations spurred on by UNESCO signed a formal act constituting the International Union for the Protection of Nature (IUPN). The initiative to set up the new organization came from UNESCO and especially from its first director general, the British biologist Julian Huxley.

At the time of its founding IUCN was the only international organization focusing on the entire spectrum of nature conservation (an international organization for the protection of birds, now BirdLife International, had been established in 1922).

IUCN (International Union for conservation of Nature) started out with 65 members in Brussels and was closely associated to UNESCO. They jointly organized the 1949 Conference on Protection of Nature in Lake Success, New York and drafted the first list of gravely endangered species. In the early years of its existence IUCN depended almost entirely on UNESCO funding and was forced to temporarily scale down activities when this ended unexpectedly in 1954. IUCN was successful in engaging prominent scientists and identifying important issues such as the harmful effects of pesticides on wildlife but not many of the ideas it developed were turned into action. This was caused by unwillingness to act on the part of governments, uncertainty about the IUCN mandate and lack of resources. In 1956, IUCN changed its name to International Union for Conservation of Nature and Natural Resources.