Great Teacher Onizuka (グレート・ティーチャー・オニヅカ Gurēto Tīchā Onizuka^?), officially abbreviated as GTO, is a Japanese shōnen manga written and illustrated by Tohru Fujisawa. It was originally serialized in Weekly Shōnen Magazine from May 1997 to April 2002. The story focuses on 22-year-old ex-bōsōzoku member Eikichi Onizuka, who becomes a teacher at a private middle school, Holy Forest Academy, in Tokyo, Japan. It won the 1998 Kodansha Manga Award for shōnen and is a continuation of Tohru Fujisawa's other manga series Shonan Junai Gumi (lit. "Shōnan True Love Group") and Bad Company, both of which focus on the life of Onizuka before he becomes a teacher in Great Teacher Onizuka.

Due to the popularity of the manga, several adaptations of GTO were established. These adaptations include a twelve-episode Japanese television drama running from July to September 1998; a live-action film directed by Masayuki Suzuki and released in December 1999; and a 43-episode anime television series aired in Japan by Fuji Television and Animax from June 1999 to September 2000. Both the anime and manga have been licensed in North America by Tokyopop. A second live action series aired in Japan during 2012.