The Big O (Japanese: THE ビッグオー, Hepburn: Za Biggu Ō) is a Japanese mecha-anime television series created by designer Keiichi Sato and director Kazuyoshi Katayama for Sunrise. The writing staff was assembled by the series' head writer, Chiaki J. Konaka, who is known for his work on Serial Experiments Lain and Hellsing. The story takes place forty years after a mysterious occurrence causes the residents of Paradigm City to lose their memories. The series follows Roger Smith, Paradigm City's top Negotiator. He provides this "much needed service" with the help of a robot named R. Dorothy Wayneright and his butler Norman Burg. When the need arises, Roger calls upon Big O, a giant relic from the city's past.

The television series was designed as a tribute to Japanese and Western shows from the 1960s and 1970s. The series is presented in the style of film noir and combines themes of detective fiction and mecha anime. The setpieces are reminiscent of tokusatsu productions of the 1950s and 1960s, particularly Toho's kaiju movies, and the score is an eclectic mix of styles and musical homages.

The Big O aired on Wowow satellite television from October 13, 1999, and January 19, 2000. The English-language version premiered on Cartoon Network's Toonami on April 2, 2001, and ended on April 23, 2001. Originally planned as a 26-episode series, low viewership in Japan reduced production to the first 13. Positive international reception resulted in a second season consisting of the remaining 13 episodes, co-produced by Cartoon Network, Sunrise, and Bandai Visual. Season two premiered on Japan's Sun Television on January 2, 2003, and the American premiere took place seven months later. Following the closure of Bandai Entertainment by parent company Bandai (owned by Bandai Namco Holdings) in 2012, Sunrise announced at Otakon 2013 that Sentai Filmworks acquired both seasons of The Big O.