ISO 639 is a standard by the International Organization for Standardization (ISO) concerned with the representation of languages and language groups. It currently consists of four sets (1–3, 5) of code, named after each part which formerly described respective set (part 4 was guidelines without its own coding system); a part 6 was published but withdrawn. It was first approved in 1967 as a single-part ISO Recommendation, ISO/R 639, superseded in 2002 by part 1 of the new series, ISO 639-1, followed by additional parts. All existing parts of the series were consolidated into a single standard in 2023, largely based on the text of ISO 639-4.

The language codes defined in the several sections of ISO 639 are used for bibliographic purposes and, in computing and internet environments, as a key element of locale data. The codes also find use in various applications, such as Wikipedia URLs for its different language editions.

The early form of ISO's language coding system was manifested by ISO/R 639:1967 titled Symbols for Languages, Countries and Authorities, which aimed chiefly to regulate vocabularies signifying languages, countries, and standardization agencies of ISO member bodies. Its "language symbols" consisted of one- or two-letter variable-length identifiers in capitalized Latin alphabets, e.g. E or En for English; S, Sp, or Es for Spanish; and In for Indonesian. It was also allowed to use (the pre-1993 version of) UDC numeral auxiliaries to indicate languages.

After decoupling the country code into ISO 3166 in 1974, the first edition of the standard ISO 639:1988 Code for the representation of names of languages was published with a framework of uniformly two-letter identifiers in lowercase Latin alphabets, mostly identical in format and vocabulary to that of the current ISO 639 Set 1.

Since then, the standard has been adopted as a fundamental technology of the rapidly expanding computer industry (RFC 1766), leading to development of more expressive three-letter framework, published as ISO 639-2:1998, largely based on MARC codes for languages. The original two-letter system was redefined as ISO 639-1 in 2001.

Seeking for more extensive support of languages for widening applications, separate supersets of the ISO 639-2 namespace that cover individual languages and groups were established as ISO 639-3 and ISO 639-5, respectively. There was also an attempt to code more precise language variants using four-letter identifiers as ISO 639-6, which was later withdrawn and to be reorganized under another framework, ISO 21636.

Relatively constant updates in parts of ISO 639 had been handled by each own authority in charge until the publication of ISO 639:2023, which harmonized and reunified the body text of former standards and brought about organizational change with a joint maintenance agency supervising all sets and issuing newsletters. The maintenance agency is located in Norway.

Each set of the standard is maintained by a language coding agency, which adds codes and changes the status of codes when needed. ISO 639-6 was withdrawn in 2014, and not included in ISO 639:2023.