The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organization devoted to the advancement of chemistry.

IUPAC was formed in 1919 by chemists from industry and academia. Over nearly eight decades, the Union has succeeded in fostering worldwide communications in the chemical sciences and in uniting academic, industrial and public sector chemistry in a common language. The Union continues to sponsor major international meetings that range from specialized scientific symposia to CHEMRAWN meetings with societal impact. During the Cold War, IUPAC became an important instrument for maintaining technical dialogue among scientists throughout the world.

It has as its members national chemistry societies. It is most well known as the recognized authority in developing standards for the naming of the chemical elements and their compounds, through its Interdivisional Committee on Nomenclature and Symbols (IUPAC nomenclature). It is a member of the International Council for Science (ICSU).

In addition to nomenclature guidelines, the IUPAC sets standards for international spelling in the event of a dispute; for example, it ruled that international aluminium is preferable to the American aluminum and American sulfur is preferable to the British sulphur.