Sephiroth (Japanese: セフィロス, Hepburn: Sefirosu) is a character and the main antagonist of Square role-playing video game Final Fantasy VII. A former soldier of the megacorporation Shinra and the coworker of Zack Fair and superior of Cloud Strife, he possesses superhuman physiology as a result of an experiment in which Shinra injected him with cells from the extraterrestrial lifeform Jenova when he was still a fetus. Upon discovering this, Sephiroth becomes consumed by rage and vengeance, decides to take control of the Planet by harnessing its life force and annihilating all life on it to become a god amidst the destruction. Sephiroth's background and role in the story are expanded upon in the Compilation of Final Fantasy VII. Additionally, he appears as a guest character in other video games and media, such as a recurring boss in the Kingdom Hearts series and as a playable character in Super Smash Bros. Ultimate.

Character designer Tetsuya Nomura conceived and designed Sephiroth as an antagonist and foil to Cloud, as he rejects his identity to fulfill what he believes to be his destiny, while Cloud re-affirms his self-worth amidst a similar crisis of identity. In Japanese, Sephiroth has been voiced by voice actor Toshiyuki Morikawa in the majority of his appearances. He was voiced by Shin-ichiro Miki in Ehrgeiz and Yūichirō Umehara as a young Sephiroth in Ever Crisis. In English, Sephiroth has been voiced by Lance Bass in Kingdom Hearts (2002), and by George Newbern in Kingdom Hearts II (2005) and the Compilation of Final Fantasy VII metaseries (2004–present). In Final Fantasy VII Remake (2020), he was voiced by Tyler Hoechlin, a role he reprises in the remastered Crisis Core: Final Fantasy VII Reunion (2022), replacing Newbern's English performance in the original release, as well as Final Fantasy VII Rebirth (2024).

Sephiroth has been well-received within the video game community and is highly ranked on many lists of the best video game villains and Final Fantasy characters based on his role in the narrative and his high challenge level. He was also the subject of analysis as a Final Fantasy villain who lost his humanity upon learning of his alien heritage, and how he attempts to corrupt Cloud by exploiting his mental weaknesses.

Sephiroth was a collaborative effort between Yoshinori Kitase, Tetsuya Nomura, and scenario writer Kazushige Nojima. Sephiroth was a heroic SOLDIER operative just like Cloud. Kitase further said Sephiroth is not a copy of one character, but taking different aspects and merging them together resulted in the character shown in the game. In regards to his characterization, the character does not see himself as an evil being but instead a chosen man to accomplish a goal; changing the world throughout his chaos will reach the Promised Land, which is the opposite of what Cloud's group wants.

Sephiroth was designed by Final Fantasy VII's character designer Tetsuya Nomura. Nomura has stated that Sephiroth was made to be a contrast to the game's main protagonist, Cloud, who was originally designed to have slicked-back, black hair. Kitase believes Sephiroth's role in Final Fantasy VII to be one of the main reasons for the game's popularity. His name was derived from the Kabbalah, in which the ten sephirot on the Tree of life represent the ten attributes through which God reveals himself. His character and role as main antagonist existed from the earliest stages of development, as originally, Nomura thought that the game's plot would deal exclusively with Cloud Strife pursuing him. His weapon, the "Masamune" (正宗), which has been featured in numerous Final Fantasy titles, is an elongated nodachi that he learned to use during his days in SOLDIER. The Masamune is named after the famous Japanese swordsmith Goro Nyudo Masamune, whose blades are considered national treasures in Japan. When designing Cloud and Sephiroth, Nomura was influenced by his view of their rivalry mirroring the legendary animosity between Miyamoto Musashi and Sasaki Kojirō, with Cloud and Sephiroth being Musashi and Kojirō respectively. Sephiroth's look was defined as "kakkoii", a Japanese term combining good looks with coolness. Yoshitaka Amano, who had handled character illustrations for previous Final Fantasy titles, noted a contrast between Cloud, a "young, passionate boy", and Sephiroth, a "more mature and cool" individual, as "intriguing", though not unusual as a pairing.

Nomura wanted Sephiroth to appear early on in the game, and then have the plot deal with the protagonists following him, so that gamers would not meet the final boss until extremely late in the game. Sephiroth was initially going to be Aerith Gainsborough's sibling, as suggested by Nomura and indicated by their similar hairstyles. Later, however, Nomura suggested he was to be changed to Aerith's past love, whom she would remember upon meeting Cloud. This character was then changed to Zack Fair, and Sephiroth's prior relationship with Aerith that was suggested by Nomura was dropped. In early drafts of the game, Sephiroth's personality was brutal and cruel, with a strong-willed and calm ego. He was to suffer from Mako addiction, resulting in a semi-conscious state as a result of high exposure to Mako energy. Sephiroth was also intended to manipulate Cloud into believing that he was a creation of his will, but this aspect of the story was later abandoned. In another cut scene, when Sephiroth's physical body is first seen in the Northern Crater, it was to be female. Jason Greenberg, the only artist working on the original PC port, recalled a crash bug that happened during Sephiroth's Super Nova (スーパーノヴァ, Sūpānova) technique. Near the end of the development cycle, many team members were done with their work and helped to test the game as much as possible. Greenberg spent nearly 24 hours playtesting battle during development. Since his transformation into Safer Sephiroth in the game's final battle, Sephiroth has had a single black wing on his back, referencing his theme song "One Winged Angel".

Nomura has called Sephiroth "the ultimate antagonist in the Final Fantasy VII saga, there can't be anyone else", and regards him as an enemy from a previous generation, in contrast to his "Remnants" who appear in Final Fantasy VII: Advent Children. For Advent Children, the film sequel to Final Fantasy VII, scriptwriter Kazushige Nojima thought that the film's plot would be less entertaining without Sephiroth. His revival in the film was introduced in the early stages of development, but the official decision as to how to bring him back was not reached until later. Nomura originally planned to have him appear from the start, but as it took the staff two years to develop his design, the idea of his presence throughout the film was scrapped, and it was decided instead to have him only appear onscreen for a short time. Sephiroth was designed for the film to emphasize his other-worldliness, such as the fact that he is never seen blinking or breathing, and his voice is always monotone and calm. In the film, the staff stated that his strength had considerably increased, to the point that he had "ascended to a new level of existence".

Despite initially encountering problems as to who would voice him, Nomura said that once Toshiyuki Morikawa auditioned for the role, they knew they had their actor. Morikawa was instructed by the staff to speak all of Sephiroth's dialogue as if he felt superior to every other character in the film. The voice director and Morikawa agreed to make Sephiroth's voice sound calm to the point that he believes he cannot lose to Cloud, suggesting to Morikawa that he may reappear at some point in the future. In the prequel Crisis Core, Morikawa portrayed Sephiroth in a charismatic fashion, as early on Sephiroth is presented as a hero. The black wing that Sephiroth and the other Shinra experiments Angeal and Genesis show act as a sign of corruption.