Sosuke Aizen (Japanese: 藍（あい）染（ぜん） 惣（そ）右（う）介（すけ）, Hepburn: Aizen Sosuke) is a fictional character in the Japanese manga and anime series Bleach created by Tite Kubo. He is the main antagonist of the first part of the story of Bleach. Serving as captain of the Fifth Division of Soul Reapers when he is introduced, Aizen later fakes his death and horribly betrays the Soul Society in search of immense power to overthrow and defeat the Soul King, before Ichigo Kurosaki and Kisuke Urahara defeat him in the newest iteration. His former lieutenant is Momo Hinamori.

Aizen spent his life working in the Fifth Division, eventually becoming its lieutenant under the former captain, Shinji Hirako. Aizen gained followers, including Gin Ichimaru and Kaname Tōsen. A century before the series' setting, after Kisuke Urahara created the Hōgyoku, an item he believed creates Soul Reaper and Hollow hybrids, Aizen began to experiment using the Soul Reapers that would become the Vizards. Among his test subjects was Shinji, who knew about Aizen, but learned too late that Aizen used his mistrust against him. Having also framed Kisuke for the Vizards' creation and forced his exile, Aizen learned the Hōgyoku actually grants its user's desires if they have the will to carry it out. But as Kisuke took the Hōgyoku with him, Aizen bides his time to take the item when the time comes.

At some point, Aizen saw the potential of Shuhei Hisagi, Momo Hinamori, Izuru Kira and Renji Abarai, and Aizen took the latter three into his division. Aizen continued experimenting on Soul Reaper/Hollow hybrids with his creations Metastacia and White, playing a respective role between Rukia Kuchiki and those of Isshin Shiba and Masaki Kurosaki. After leaving the Soul Society, Aizen, Ichimaru and Tosen travelled to Hueco Mundo and arranged an alliance with the self-proclaimed god king of Hueco Mundo, Barragan Luisenbarn. As Barragan refused, Aizen revealed all of the Hollow minions were killed during the discussion and forces Barragan into servitude. From there, Aizen appropriates Barragan's palace Las Noches and reconstructs it in his own image, recruiting Tier Halibel and other Arrancars that would become his Espadas.

Aizen is an extremely powerful and intelligent combatant, well above many captain-level Soul Reapers, which is the highest-ranking position just under the head captain. When preparing to activate the Hōgyoku, he states that despite its half-awakened state, it can be fully activated temporarily when fusing with someone with at least twice as much spirit pressure as an average captain-level Soul Reaper, referring to himself. After Aizen embedded the Hōgyoku into his body, he undergoes a series of transformations that give him extraordinary boosts in his strength and power. There are very few that can battle him and survive.

Originally, Aizen carried a zanpakutō called Kyōka Suigetsu (鏡花水月; literally "flower in the mirror, moon on the water"). Kyōka Suigetsu's Shikai ability Kanzen Saimin (完全催眠; literally "Complete Hypnosis"), activated with the command "shatter" (砕けろ, kudakero), is effective if the target watches the Shikai sequence once and thus can fall under Aizen's deception any time he wishes. There are only a few means to escape the sword's influence such as touching the blade before it is activated as Gin overheard. Though unaware of that fact, in his gambit to take down Aizen, Yamamoto grabbed the blade when it pierced him and read the blade's force to distinguish Aizen. After his final evolution by the Hōgyoku, he was made immortal. It was only thanks to Ichigo's Mugetsu technique that Aizen was able to be sealed. Afterwards, Aizen finds that he no longer needs Kyōka Suigetsu itself as the weapon completely dissolves with its powers now a part of his being.

Setting up the events from the shadows, Aizen arranged for Rukia Kuchiki to meet with Ichigo Kurosaki so he can become a Soul Reaper. From there, by sending Hollows in Karakura Town to alert the Soul Society for the location to get the Hōgyoku that is concealed within her body, Aizen created an elaborate conspiracy among the Thirteen Court Squad captains by secretly killing the members of Central 46 Chambers and using his Zanpakutō to create an illusion of them and sentence Rukia to death. Gin fakes Aizen's death to have the captains and lieutenants fighting against themselves. However, Squad 4 Captain Retsu Unohana uncovered the truth, as Aizen removed the Hōgyoku from Rukia, before he, Gin and Tōsen escape into Hueco Mundo. There, Aizen prepares his goal: Using the Hōgyoku to sacrifice the souls of Karakura Town in order to create an Ōken (王鍵) and kill the Soul King.

From there, Aizen begins to make several Arrancars in his service stronger using the Hōgyoku and sends some of his Espadas to the Land of the Living to gain information on Ichigo. Eventually, claiming to have an interest in her power, Aizen has Orihime Inoue brought to Las Noches on the notion that he needs her to save the powers of the deteriorating Hōgyoku. However, sacrificing the weakest members of the Espadas in the process, Aizen revealed the kidnapping as a distraction for Ichigo and as part of a scheme to cut the Soul Society's forces in half by the time he enacts his attack on Karakura Town. Though he came with three of his strongest Espadas, the defeat of Staark and Baraggan convinces Aizen that he has surpassed his own minions and strikes Halibel down to deal with the opposition personally. When confronted by Ichigo, who is the only person not affected by his Zanpakutō, Aizen reveals his role in the youth's life and struggles since meeting Rukia. Aizen then almost reveals Ichigo's lineage when Isshin intervened to take over the fight. Aizen reveals that he absorbed the Hōgyoku as it grants his desire for a Hollow-like form to easily defeat his enemies.

Aizen makes his way to the Soul Society so that he can create the Ōken, finding Ichigo's school friends and deciding to kill them first before enacting his goal. Aizen is wounded by Gin, who reveals he tricked Aizen by pretending to be his right-hand man until the time was ideal to betray him. However, his will to survive answered by the Hōgyoku, Aizen evolves into another form and mortally wounds Gin as Ichigo arrives. Though expecting him not to be a threat due to his sudden lack of spirit energy, Aizen finds himself overwhelmed by Ichigo's newly enhanced physical strength. Though his fear caused the Hōgyoku to turn him into a Hollow-like monster, Aizen barely survived Ichigo's final attack as he evolved once more to the point that he became immortal. However, Ichigo's attack weakened and defeated Aizen to the point that the Hōgyoku no longer acknowledges him as he is reverted to his human form while engulfed in a specially made kidō "Seal" that Urahara planted on him. Ten days later, numerous seals placed on his body to render him immobile and to keep his now vast reiatsu around his body, Aizen is sentenced by Central 46 to 18,800 years of imprisonment in the lowest underground prison, the 8th prison known as "Muken". Aizen's arrogance towards the matter results with him being gagged as the final seals are added and the sentence escalated to 20,000 years.