Xenosaga[a] is a role-playing video game series developed by Monolith Soft and primarily published by Namco.

Forming part of the wider Xeno metaseries, Xenosaga is set in a science fiction universe and follows a group of characters as they face both a hostile alien race called the Gnosis and human factions fighting for control of the Zohar, an artifact connected to a god-like energy called U-DO.

Gameplay across the series is similar, with the characters being guided through a linear narrative and fighting enemies using a turn-based combat system.

Following the release of the first game, the Xenosaga series was given over to new staff with Takahashi both supervising the project and providing the draft scripts.

Following the end of the Xenosaga series, Takahashi and other team members started a new project to rebuild morale, which became Xenoblade Chronicles.

[17][18] Freaks was part of a movement with the Xenosaga series to turn it into a multimedia franchise, with the project growing substantially larger than previously planned.

Combat makes use of a traditional turn-based battle system, with basic mechanics involving the spending or conserving of Action Points (AP).

[30] Episode II incorporates multiple levels of attack types which different enemies are weak to, combined with alterations to the performance of mechs.

[33][34] Pied Piper incorporates gameplay systems similar to Episode I adjusted for mobile devices, with navigation separate from battles which take place in a virtual zone tied to the narrative.

Beginning as a war between the U-TIC Organization and the Federation, it escalated when a group of Realians went berserk when an experiment to suppress U-DO's energies went wrong.

[38][39] The first game follows series protagonists Shion Uzuki and prototype battle android KOS-MOS escaping a Gnosis attack and traveling to Second Miltia aboard the passenger freighter Elsa, where they meet a young man called chaos.

Ormus' leader Sergius gains control of a powerful mech called Proto Omega, destroying Miltia in the process.

[41] In Episode III, Shion, KOS-MOS, chaos and the rest of their group confront both the Testaments—which includes a resurrected Albedo—and Vector CEO Wilhelm.

[42] Pied Piper follows the character Jan Sauer—later known as the cyborg Ziggurat 8 (Ziggy for short)—as he investigates the crimes of a cyberspace-based serial killer a century before the events of Episode I.

[11][43][44][45] The creator of Xenosaga was Tetsuya Takahashi, who had previously worked at Square on multiple projects including entries in the Final Fantasy series.

In addition to up the story of Shion while also leaving room for further entries if there was enough demand, further adjustments were made to both gameplay and graphics based on combined staff comments and fan feedback.

[19][57] The Xenosaga series incorporates multiple references to Biblical mythology, Jewish mysticism and Gnosticism; several character, object and place names (such as Nephilim, Zohar, Gnosis, Merkabah) are taken from sources within these belief systems.

Hosoe and Saso worked together as part of the Super Sweep music group, but they did not collaborate with Kajiura on any tracks, or even meet with her during the game's production.

He also noted that Xenosaga showed a trend with Takakashi's work of being ambitious to the point of needing to compromise the original plan due to external limitations.

The narrative was generally praised for its complex structure and cinematic approach, while its gameplay was seen as enjoyable despite initially being complicated even by genre standards and the mech segments not feeling consequential.

[69][78][79][80][81][82] Episode III received praise for its narrative and the way it concluded the series' overarching plot, but many felt that there was too much need for knowledge of the original games for it to be enjoyable for newcomers.

[33][34][72] Episode I managed a strong debut, selling over 240,000 units within three days of its release,[88] and becoming the seventh best-selling game in Japan during 2002.

[102] Following the release of Episode III, and the mixed reception received by the series as a whole, the entire development team were in a state of low morale.

[107][108] KOS-MOS and her rival from Xenosaga Episode III T-elos appeared in Super Robot Taisen OG Saga: Endless Frontier.

[109][110] Supporting character MOMO was later introduced alongside KOS-MOS and T-elos in the game's sequel Super Robot Taisen OG Saga: Endless Frontier Exceed.

[111][112] Both KOS-MOS and T-elos starred in Project X Zone for the Nintendo 3DS, a spiritual successor to Namco × Capcom from the same development team.

KOS-MOS's head armor from Xenosaga I makes a late game appearance in Tales of Arise as part of its artifacts system.