Xenosaga 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. The party fights both on foot and in a variety of mechs.

Tetsuya Takahashi created Xenosaga as a spiritual successor to the Square-produced Xenogears, for which he founded Monolith Soft with help from Namco; multiple Xenogears staff returned, including co-writer Soraya Saga. 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. Under the new staff, the original script saw several changes and its planned six-part structure cut down by half. The series made considerable use of Biblical mythology and elements of the works of Carl Jung and Friedrich Nietzsche, with the subtitles of the main trilogy drawing from the works of Nietzsche.

Reception of individual titles has been positive, although journalists have commented that the series was too ambitious. While the first game met with strong sales, the series as a whole was a commercial disappointment. The first game also received both a manga and an anime adaptation, the latter being dubbed and released in North America. Following the end of the Xenosaga series, Takahashi and other team members started a new project to rebuild morale, which became Xenoblade Chronicles. Characters from Xenosaga would go on to appear in multiple crossover games.