X.25 is the name given to a suite of protocols used for packet-switched wide area network communication. Defined by the International Telegraph and Telephone Consultative Committee in 1976, X.25 had the original purpose of carrying voice signals over analog telephone lines.

X.25 is the oldest packet-switching technique available and was commonly used before the Open System Interconnection (OSI) reference model became standard. Originally developed for use in the 1970s and used widely in the 1980s, X.25 has since fallen out of favor, having been replaced by less complex protocols such as Internet Protocol. Today, it is mostly relegated to ATMs and credit card verification networks.