The Xenosaga (ゼノサーガ) series is a series of science fiction role-playing video games developed by Monolith Soft and published by Namco Bandai on the PlayStation 2. The series began with the 2002 release of Episode I: Der Wille zur Macht, which was followed in 2004 by Episode II: Jenseits von Gut und Böse and in 2006 by Episode III: Also sprach Zarathustra. The music of Xenosaga includes the soundtracks to all three chapters, as well as the music for its spin-off media. These include Xenosaga Freaks, a set of minigames set between the first two chapters, Xenosaga I & II, a Nintendo DS remake of the first two chapters, and Xenosaga: The Animation, an anime series covering the events of the first game. Episode I was composed by Yasunori Mitsuda, Episode II was split between Yuki Kajiura, Shinji Hosoe, and Ayako Saso, and Episode III was composed by Kajiura alone. Xenosaga Freaks reused some of the music of Episode I by Mitsuda while also including new work by Shinji Hosoe, Ayako Saso, Masashi Yano, Keiichi Okabe, Satoru Kōsaki, and Hiroshi Okubo, while Xenosaga I & II and Xenosaga: The Animation used new pieces composed by Kousuke Yamashita.
Episode I sparked the release of two versions of its soundtrack album as well as a single, Episode II received two soundtrack albums, and Episode III received one. Xenosaga Freaks and Xenosaga: The Animation each also sparked the release of a soundtrack album. In addition to the albums, Episode I saw the release of a book of orchestral sheet music as well as a book of piano sheet music; Episode II had only a book of piano sheet music. Music from the series has been played multiple times in concerts, especially that of the first two games.