Great Teacher Onizuka, officially abbreviated as GTO, is a Japanese manga series written and illustrated by Tooru Fujisawa. It was originally serialized in Kodansha's Weekly Shōnen Magazine from January 1997 to February 2002, with its chapters collected in 25 tankōbon volumes. 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 is a continuation of Fujisawa's earlier manga series Shonan Junai Gumi and Bad Company, both of which focus on the life of Onizuka before becoming a teacher.

Due to the popularity of the manga, several adaptations of GTO were created, including 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 produced by Pierrot, which aired in Japan on Fuji TV from June 1999 to September 2000. A second live-action series aired in Japan during 2012, and two more in 2014.

A sequel manga series, titled GTO: 14 Days in Shonan, ran in Weekly Shōnen Magazine from June 2009 to September 2011. Another sequel, titled GTO: Paradise Lost, began in Weekly Young Magazine in April 2014. Both the anime and manga were licensed in North America by Tokyopop. The anime series was re-licensed by Discotek Media in 2012. The manga is licensed by Kodansha USA.

The manga has had over 50 million copies in circulation, making it one of the best-selling manga series in history. In 1998, Great Teacher Onizuka won the 22nd Kodansha Manga Award in the shōnen category.