Metastaseis (Greek: Μεταστάσεις; spelled Metastasis in correct French transliteration, or in some early writings by the composer Métastassis) is an orchestral work for 61 musicians by Iannis Xenakis. His first major work, it was written in 1953–54 after his studies with Olivier Messiaen and is about 8 minutes in length. The work was premiered at the 1955 Donaueschingen Festival with Hans Rosbaud conducting. This work was originally a part of a Xenakis trilogy titled Anastenaria (together with Procession aux eaux claires and Sacrifice) but was detached by Xenakis for separate performance.

Metastaseis requires an orchestra of 61 players (12 winds, 3 percussionists playing 7 instruments, 46 strings) with no two performers playing the same part. It was written using a sound mass technique in which each player is responsible for completing glissandi at different pitch levels and times. The piece is dominated by the strings, which open the piece in unison before their split into 46 separate parts.

A ballet was choreographed to Xenakis' Metastaseis and Pithoprakta by George Balanchine (see Metastaseis and Pithoprakta). The ballet was premiered on January 18, 1968 by the New York City Ballet with Suzanne Farrell and Arthur Mitchell.