Shin Megami Tensei: Nine was published by Atlus and released on Xbox, exclusively in Japan, in 2002.

Forming part of the Megami Tensei series, Nine takes place in the period of time between Shin Megami Tensei and its sequel, with the survivors of Tokyo's destruction sheltering in underground bunkers. Taking on the role of a debugger, the player navigates the Idea Space virtual world set up by the survivors of Tokyo's destruction, which has come under attack by demonic beings called "noise". The gameplay has the player navigating a customized avatar in third-person through the virtual world of Tokyo, battling enemies using a real-time command-based battle system. Despite its title, it is the eighth game in the Megami Tensei series. The game's title instead refers to the number of possible moral alignments available to players.

Nine is set in the year "202X", taking place during the latter half of Shin Megami Tensei and in the time leading up to its sequel Shin Megami Tensei II. In the wake of the ICBM attack on Tokyo, communities of survivors now live in underground cities. In the face of their predicament, the underground inhabitants create "Idea Space", a virtual world based on Tokyo from the 1990s. The protagonist, named Kei Azuma by default, is exploring the virtual world when "Noise", digitized demons hostile to humans, appear and begin attacking Idea Space players through their avatars. After fending off an attack, Azuma is employed by the Central Administration Bureau as a Debugger, a player who traverses Idea Space and repels Noise invasions. During their missions through Idea Space, Azuma meets with multiple characters: Baraki/Sumire, a childhood friend who represents "Chaos", Mubiora/Miranda, a representative from the Central Administration Bureau who represents "Law". While going through their duty as a Debugger, the events of the latter half of Shin Megami Tensei take place, which culminate in all parts of Tokyo being destroyed in a Great Flood triggered by the God-worshipping Order of Messiah: Azuma survives due to their body being in cold storage, meaning that their mind becomes linked to Idea Space along with their Law and Chaos allies.

During a mission into the virtual Shibuya 109 against the Noise demon Abaddon, the true origins of Idea Space are revealed. Idea Space was created by humans tired of awaiting the arrival of their "Messiah" in the real world: hoping to bring about salvation through technology, the humans created the Idea Space, and generated Maria as its goddess and administrator to further the creation of Millennial Kingdom by manipulating its users. On their mission into Shibuya 109, Azuma retrieves a Desire Disc, an artifact containing the saved desires of humanity that is the key to both sides' plans. Azuma is faced with choices about which faction to support. The Law faction, supported by Maria, wishes to bring peace to humanity along with releasing the Idea Space's inhabitants. The Chaos faction, which can be led by Lucifer if he is freed from his prison within a metaphysical plain called the Expanse, intends to destroy the inhabitants of Idea Space and start anew with the chosen Adam and Eve: Azuma and the Chaos companion. To fulfill any of their chosen roles, Azuma must defeat Yaldabaoth, the controller of Idea Space who is keeping the system going and will not allow either Maria or Lucifer to carry out their plans.