UTF-8 is a variable-length character encoding standard used for electronic communication. Defined by the Unicode Standard, the name is derived from Unicode (or Universal Coded Character Set) Transformation Format – 8-bit.

UTF-8 is capable of encoding all 1,112,064 valid character code points in Unicode using one to four one-byte (8-bit) code units. Code points with lower numerical values, which tend to occur more frequently, are encoded using fewer bytes. It was designed for backward compatibility with ASCII: the first 128 characters of Unicode, which correspond one-to-one with ASCII, are encoded using a single byte with the same binary value as ASCII, so that valid ASCII text is valid UTF-8-encoded Unicode as well.

UTF-8 was designed as a superior alternative to UTF-1, a proposed variable-length encoding with partial ASCII compatibility which lacked some features including self-synchronization and fully ASCII-compatible handling of characters such as slashes. Ken Thompson and Rob Pike produced the first implementation for the Plan 9 operating system in September 1992. This led to its adoption by X/Open as its specification for FSS-UTF, which would first be officially presented at USENIX in January 1993 and subsequently adopted by the Internet Engineering Task Force (IETF) in RFC 2277 (BCP 18) for future internet standards work, replacing Single Byte Character Sets such as Latin-1 in older RFCs.

UTF-8 results in fewer internationalization issues than any alternative text encoding, and it has been implemented in all modern operating systems, including Microsoft Windows, and standards such as JSON, where, as is increasingly the case, it is the only allowed form of Unicode.

UTF-8 is the dominant encoding for the World Wide Web (and internet technologies), accounting for 97.9% of all web pages, over 99.0% of the top 10,000 pages, and up to 100.0% for many languages, as of 2023. Virtually all countries and languages have 95.0% or more use of UTF-8 encodings on the web.