Giannis Klearchou Xenakis (also spelled for professional purposes as Yannis or Iannis Xenakis; Greek: Γιάννης "Ιωάννης" Κλέαρχου Ξενάκης, pronounced [ˈʝanis kseˈnacis]; 29 May 1922 – 4 February 2001) was a Romanian-born Greek-French avant-garde composer, music theorist, architect, performance director and engineer.

After 1947, he fled Greece, becoming a naturalised citizen of France eighteen years later. Xenakis pioneered the use of mathematical models in music such as applications of set theory, stochastic processes and game theory and was also an important influence on the development of electronic and computer music. He integrated music with architecture, designing music for pre-existing spaces, and designing spaces to be integrated with specific music compositions and performances.

Among his most important works are Metastaseis (1953–54) for orchestra, which introduced independent parts for every musician of the orchestra; percussion works such as Psappha (1975) and Pléïades (1979); compositions that introduced spatialization by dispersing musicians among the audience, such as Terretektorh (1966); electronic works created using Xenakis's UPIC system; and the massive multimedia performances Xenakis called polytopes, that were a summa of his interests and skills.

Among the numerous theoretical writings he authored, the book Formalized Music: Thought and Mathematics in Composition (French edition 1963, English translation 1971) is regarded as one of his most important publications. As an architect, Xenakis is primarily known for his early work under Le Corbusier: the priory of Sainte-Marie de La Tourette, on which the two collaborated, and the Philips Pavilion at the 1958 Brussels World's Fair (Expo 58), which Xenakis designed by himself.