Chaos;Head (stylized as ChäoS;HEAd) is a 2008 science fiction visual novel video game developed by 5pb. and Nitroplus. It is the first game in the Science Adventure series. An enhanced version titled Chaos;Head Noah was released for Xbox 360 in 2009, and has since been ported to multiple platforms. An English localization was released by Spike Chunsoft for Nintendo Switch and Windows in 2022. The game follows Takumi Nishijou, who gets involved in the "New Gen" serial murder case. He frequently experiences delusions and hallucinations, some of which the player can influence, which affects the progression of the story.

The game was planned by Chiyomaru Shikura and written by Hayashi Naotaka, and features character designs by Sasaki Mutsumi and music by Takeshi Abo, who described the game's sound as "rainy". The game was commercially successful, helping 5pb. establish itself as a game developer, and was well received for its story. In addition to the direct sequel Chaos;Head Love Chu Chu! and the thematic sequel Chaos;Child, manga, an anime series, and an internet radio show based on Chaos;Head have been produced.

The game is set in 2009 in the Shibuya district of Tokyo, where Takumi lives in a cargo crate on top of an apartment building. He discusses the recent "New Generation Madness" ("New Gen") serial murder case in Shibuya with his friend Grim over the internet, when someone with the username "Shogun" sends Takumi image files depicting a man pinned to a wall with stakes. Later, Takumi witnesses a girl he does not recognize committing the murder portrayed in the image files, and he flees the murder scene. A few days later, she sits next to him in school. He thinks she will kill him, but is told that they supposedly have been friends for a year and that her name is Rimi Sakihata.

Convinced that Shogun is targeting him, Takumi tries to avoid getting involved in the murder case, which draws the attention of the police. As he becomes a suspect and more murders occur, Takumi worries that Shogun is targeting him and experiences paranoia and hallucinations, and becomes unsure of what is real and who he can trust. In one of his delusions, Shogun appears as an old man in a wheelchair and tells him that more people will die unless he awakens.

When Takumi sees a girl carrying a large sword in public and notices that only he can see it, Ayase Kishimoto – a student who recently transferred to Takumi's school – tells him that he needs a "DI-sword" to be saved. She makes such a sword materialize in her hands, and tells him about how Takumi, the girl carrying the sword – Sena Aoi – and her friend Kozue Orihara are Gigalomaniacs: people with the ability to project delusions into reality, which is called "real-booting".

The Nozomi Group, a technology company in Shibuya, is revealed to be using their Noah II machine to synthetically use Gigalomaniac power, and have staff carrying transmitters to increase Noah II's signal reach.

Takumi learns that he himself is a projected delusion with fabricated memories: a copy created by Shogun, who is the original Takumi. His sister Nanami is held in the location of the real Noah II, where she is made to awaken as a Gigalomaniac and obtain a DI-sword. Rimi – also a Gigalomaniac – tries to save her, but is attacked by Norose and taken prisoner in Nanami's stead. Shogun again meets with Takumi to tell him that he had intended to stop Nozomi from taking over humanity with Noah II, but that his body – aged from disease and overuse of Gigalomaniac powers – hindered him, prompting him to create Takumi to do it in his stead. Takumi obtains a DI-sword, and destroys the transmitters, revealing the nurse Hazuki to be the New Gen killer in the process by reading her memories and projecting them onto the screens on the buildings. An earthquake occurs, destroying much of Shibuya; Takumi rescues the other Gigalomaniacs from the destruction, and continues to the location of Noah II. He fights Norose, but Noah II overwhelms him with delusions; he reaffirms his existence with the help of the other Gigalomaniacs' delusion synchronization, and destroys Norose and Noah II with his DI-sword. Lying in the ruins of Shibuya, depending on what the player chooses during the synchronization, he either gives up his life along with the dying Shogun's (in the "A" route), or stays alive with Rimi (the "AA" route). In Chaos;Head Noah, players are locked onto the "Silent Sky" route the first time they play the game – similar to the original's "A" route – and it also features additional routes dedicated to the female main cast. All of these routes must be completed in order to unlock the "Blue Sky" route, which is similar to the original's "AA" route.