Seishiro Sakurazuka (Japanese: 桜塚 星史郎, Hepburn: Sakurazuka Seishirō; November 22, 1965 - c. 1999) is a fictional character that was created by Clamp and first appeared in the manga series Tokyo Babylon. He appears as a kind veterinarian who shows romantic affection for the young exorcist (referred as onmyōji) Subaru Sumeragi. During the story, it is revealed he is the Sakurazukamori (桜塚護; lit. "Cherry Blossom burial mound guardian"), Japan's number-one assassin who is targeting Subaru. The character returns in the apocalyptic manga X as a combatant in the battle of Armageddon, a Dragon of Earth and Subaru's rival. An alternative version of Seishiro appears in Clamp's crossover series Tsubasa: Reservoir Chronicle as a young treasure hunter who is searching for two vampires, one of whom is an alternative version of Subaru.

Seishiro was created alongside Subaru and his twin sister Hokuto as the main characters for a dōjinshi novel. After undergoing major changes, he was used as a main character in Tokyo Babylon. Multiple actors voiced the character in animated adaptations of the manga. Since his introduction in Tokyo Babylon, publications for manga and anime have received Seishiro favorably for his relationship with Subaru, which became one of the most famous rivalries in X. His role in Tsubasa: Reservoir Chronicle also received positive responses for his action scenes and connections with the protagonist Syaoran.

Seishiro, with twin characters Subaru and Hokuto, were created by series creator Nanase Ohkawa for a dōjinshi novel about an onmyōji who hunts elves. The characters were twice drawn for covers; these characters were used when South magazine asked Clamp to create a new series for them. In the dōjinshi, Seishiro was a veterinarian but his dark side was much less pronounced. Seishiro underwent more changes from this early design to the creation of Tokyo Babylon. Seishiro was easier to write than Subaru by Ohkawa as she has more skill with coldhearted characters like Seishiro's true persona than the kind Subaru. In the dōjinshi, Seishiro had a conjure being accompanying him (shikigami) which was named Nandaro ("Just what is this?"). Clamp privately referred to it by this name during the publication of the series despite it not being mentioned in the manga. According to Yasunori Katō, Subaru and Seishiro's respective designs pay homage to this character and the protagonist of the fantasy novel Teito Monogatari, who is widely credited with starting the "onmyōji boom" in Japan. The early chapters involving Subaru's meeting with Seishiro set the events of the series' future. Since his introduction, Seishiro has been written with the idea of having an agendum that contrasts with his actions and dialogue with the other characters. When designing Seishiro, Clamp wanted a strong contrast between his facade as a caring man and his cruel persona whenever he is wearing glasses. The Clamp News 2 newsletter lists Seishiro's birth as taking place in Kanazawa at 6am on November 22, 1965.

When production of Tokyo Babylon was finished, film director George Iida asked Clamp to help with a sequel movie in which Subaru and Seishiro would clash again. Because of production issues, the characters' interactions were reduced and were used only in an audio drama. To promote the movie, Mokona drew illustrations of both characters. Subaru and Seishiro appear in X because their relationship parallels those of Kamui Shiro and Fuma Monou. A common theme involves the series' fate; Subaru expresses no interest in the future of the Earth, but he and his counterpart are still drawn to Tokyo on the Promised Day. As a result, a fatalistic atmosphere persists in the series. Ohkawa found Seishiro to be childish because he only deals with what appeals to him; in contrast, Ohkawa found Fuma more adult-like. In the sixteenth volume of the series, the rivalry between the two ended with Seishiro's death; according to Clamp Seishiro's death is permanent and he would not return in future chapters.

In Tsubasa: Reservoir Chronicle, Clamp originally intended Subaru to be the mentor of the protagonist, Syaoran, while searching for Seishiro. However, they ended replacing him with Seishiro to avoid repeating the same pattern Tokyo Babylon and X had regarding these two characters.

In the Tokyo Babylon original video animation, Seishiro was first voiced by Takehito Koyasu, who was replaced by Tōru Furusawa for the 1996 film X. Otoya Kawano took the role in the television adaptation. Hiroki Tōchi played Seishiro in Tsubasa: Reservoir Chronicle, and the character will be voiced by Yūichirō Umehara in the television series Tokyo Babylon 2021.

In English, Seishiro was first voiced by Dean Fenton in Tokyo Babylon, by Garrick Hagon in the feature film X, Dave Mallow in the X television series, and J. Michael Tatum in Tsubasa.

In Clamp no Kiseki, the writers described the character's past with Subaru revealed in flashback foreshadow an important event in his life. As Seishiro constantly mentions his one-sided affections towards him, Subaru often notices he also cares about him. Due to the dark nature Seishiro hides from Subaru, the two can be regarded as the yin and yang as the former contrasts his the latter's kind self even when interacting about his job. Subaru's hidden romantic feelings towards Seishiro become obvious as the manga progresses as he becomes astonished when Seishiro keep profressing his love towards him. Although Subaru and Seishiro are written as rivals, illustrations from the X series tend to show the bond the two had. Clamp believes the red thread of fate they share in an image was drawn in a forced manner in restrospect. Despite his calm demeanor, Seishiro becomes confused when Fuma Monou explains to Seishiro that Subaru's true wish isn't what he thinks, hinting that Seishiro is still interested in Subaru. When Seishiro dies in battle, Fuma offers Subaru to transplant Seishiro' lost eye to Subaru. Clamp wrote it in a way so the bond between Subaru and Seishiro will never be destroyed.