The International Union of Pure and Applied Chemistry (IUPAC,  /ˈaɪjuːpæk/ EYE-ew-pak or /ˈjuːpæk/ EW-pak) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science (ICSU). The international headquarters of IUPAC is located in Zürich, Switzerland. The administrative office, known as the "IUPAC Secretariat", is located in Research Triangle Park, North Carolina, United States. This administrative office is headed by the IUPAC executive director.

IUPAC was established in 1918 as the successor of the International Congress of Applied Chemistry for the advancement of chemistry. Its members, the National Adhering Organizations, can be national chemistry societies, national academies of sciences, or other bodies representing chemists. There are fifty-four National Adhering Organizations and three Associate National Adhering Organizations. IUPAC's Inter-divisional Committee on Nomenclature and Symbols (IUPAC nomenclature) is the recognized world authority in developing standards for the naming of the chemical elements and compounds. Since its creation, IUPAC has been run by many different committees with different responsibilities. These committees all run different projects which include standardizing nomenclature, finding ways to bring chemistry to the world, and publishing works.