Seiji Sasaki is a Japanese actor and voice actor born on January 11, 1965, in Saitama Prefecture, Japan. He is affiliated with the theatre company Theatre Company Subaru and has been active in the industry since 1990. His work spans anime television series, original video animations, theatrical films, video games, tokusatsu, and dubbing roles for foreign live-action and animated productions. He is particularly known for dubbing over African-American actors in live-action content.

Sasaki has built an extensive filmography with several notable and recurring roles. In the long-running series One Piece, he voices the character Blueno, a role he has played in the main television anime, as well as in the special One Piece Episode of Merry: Mō Hitori no Nakama no Monogatari and the film One Piece Film: Red. He also provides the voice for the pirate Squard and a character named Mitsuboshi in the same franchise. Another prominent role is Sig Curtis in both the 2003 Fullmetal Alchemist series and its successor, Fullmetal Alchemist: Brotherhood. In the comedy anime Shinryaku! Ika Musume, also known as Squid Girl, he plays the recurring character Harris, a scientist convinced of the existence of aliens. He also voiced the character Bruce Irvin in the 1998 anime film Tekken: The Motion Picture.

Beyond these specific examples, Sasaki's career includes a wide variety of other roles. He is the voice of Reacoom in Dragon Ball Kai and Atsurō Kawai in the Initial D series. In the video game and anime adaptation Inazuma Eleven, he voices the antagonist Reiji Kageyama. His work also extends to tokusatsu, where he provided the voice for the villain Messiah in Tokumei Sentai Go-Busters. In animated films, his credits include roles in Howl's Moving Castle, the Eureka Seven: Hi-Evolution trilogy, and several Pokémon movies.

In addition to his anime work, Sasaki has a substantial career in dubbing foreign media. He is the regular Japanese dubbing voice for actor Christopher Judge, known for his role as Teal'c in Stargate SG-1. Other notable dubbing credits include voicing characters in the Alien Resurrection film, the James Bond films Live and Let Die and The World Is Not Enough, and the Doctor Who series.