An internationalized domain name (IDN) is an Internet domain name containing one or more non-ASCII characters. Such domain names could contain letters with diacritics, as required by many non-English languages, or characters from non-Latin scripts such as Arabic, Hebrew, Chinese or Hindi. However, the standard for domain names does not allow such characters, and much work has gone into finding a way to internationalize domain names into a standard ASCII format, thereby preserving the stability of the domain name system.

IDN has, by the standards of the Internet, a long history; it was originally proposed in 1996 (by M. Dürst) and implemented in 1998 (by James Seng under the guidance of T.W.Tan). After much debate and many competing proposals, a system called Internationalizing Domain Names in Applications (IDNA) was adopted as the chosen standard, and has been rolled out in several top level domains.

In IDNA, the term internationalized domain name means specifically any domain name consisting only of labels to which the IDNA ToASCII algorithm can be successfully applied. (For the meaning of 'label' and 'ToASCII', see the section ToASCII and ToUnicode below.) In March 2008, the IETF formed a new IDN Working Group to update the current IDNA protocol.