Xenosaga Episode I: Der Wille zur Macht (ゼノサーガ エピソードI 力への意志 Zenosāga Episōdo Wan Chikara e no Ishi?) is a role-playing video game developed by Monolith Soft and published by Namco for the PlayStation 2 and the first title in the Xenosaga series. Der Wille zur Macht, "The Will to Power", is a reference to Friedrich Nietzsche's posthumous collection of notes and unused aphorisms, which was intended to become his magnum opus. While supplies lasted, players who pre-ordered Episode I received a limited-edition artbook featuring original art from the game.

Episode I begins the trilogy that centers around the characters Shion and KOS-MOS. A young woman employed by Vector Industries, Shion aided in the creation of the battle android KOS-MOS and is present throughout much of the game. During this game, the U-TIC Organization attempts to steal one of the powerful Zohar Emulators, sources of energy used by humans.

Episode I was written and directed by Tetsuya Takahashi, director of the PlayStation game Xenogears. The Xenosaga series is often claimed to be a spiritual prequel to Takahashi's earlier game, although the game's creators deny any real connection.