The IUPAC International Chemical Identifier (InChI, pronounced /ˈɪntʃiː/ IN-chee or /ˈɪŋkiː/ ING-kee) is a textual identifier for chemical substances, designed to provide a standard and human-readable way to encode molecular information and to facilitate the search for such information in databases and on the web. Initially developed by IUPAC and NIST during 2000–2005, the format and algorithms are non-proprietary and the software is freely available under the open source LGPL license. The continuing development of the standard has been supported since 2010 by the not-for-profit InChI Trust, of which IUPAC is a member. The current version is 1.04 and was released in September 2011.

The identifiers describe chemical substances in terms of layers of information — the atoms and their bond connectivity, tautomeric information, isotope information, stereochemistry, and electronic charge information. Not all layers have to be provided; for instance, the tautomer layer can be omitted if that type of information is not relevant to the particular application.