The Mysterians (Japanese: 地球防衛軍, Hepburn: Chikyū Bōeigun, lit. 'Earth Defense Force') is a 1957 Japanese epic science fiction film directed by Ishirō Honda, with special effects by Eiji Tsuburaya. Produced and distributed by Toho Co., Ltd., it is the first Honda-Tsuburaya collaboration filmed in both color and TohoScope, and stars Kenji Sahara, Yumi Shirakawa, Momoko Kōchi, Akihiko Hirata, Yoshio Tsuchiya, Susumu Fujita, and Takashi Shimura, with Haruo Nakajima and Katsumi Tezuka as Moguera. In the film, Earth's defense forces unite to combat an extraterrestrial race that desires to intermarry with human women and settle on the planet.

Inspired by the success of big-budget science fiction films in Japan and the United States, Toho executives became keen on producing a science fiction epic of their own. Producer Tomoyuki Tanaka recruited science fiction writer Jōjirō Okami to develop the story, which Shigeru Kayama later adapted for Takeshi Kimura's screenplay. Honda stated that three companies were involved in the film's production, which was the most in any tokusatsu production that he directed.

The Mysterians was released theatrically in Japan on December 28, 1957, as a double feature with Sazae's Youth. It was a box office success in Japan upon its release, earning ¥193 million against its ¥200 million budget during its original theatrical run, making it the tenth highest-grossing Japanese film of 1957, and leading Toho to produce two further space-themed science fiction epics: Battle in Outer Space (1959) and Gorath (1962). An English dub of the film was produced by RKO Radio Pictures and distributed in the United States by Metro-Goldwyn-Mayer on May 27, 1959, where it grossed $975,000 and reportedly received mostly positive reviews. Western film critics praised Tsuburaya's special effects, but some criticized the plot as confusing and juvenile.