Invasion of Astro-Monster (怪獣大戦争, Kaijū Dai-sensō, lit. 'The Great War of the Monsters') is a 1965 kaiju film directed by Ishirō Honda, with special effects by Eiji Tsuburaya. It is the sixth film in the Godzilla franchise and Shōwa period. The film was a Japanese-American co-production; it was the second collaboration between Toho Co., Ltd. and UPA. The film stars Akira Takarada, Nick Adams, Kumi Mizuno, Akira Kubo, and Yoshio Tsuchiya, with Haruo Nakajima as Godzilla, Masaki Shinohara as Rodan, and Shoichi Hirose as King Ghidorah. In the film, aliens plead with humanity to borrow Godzilla and Rodan to defeat Ghidorah, only to betray the humans and unleash the monsters on the Earth.

Invasion of Astro-Monster was theatrically released in Japan on December 19, 1965, to mixed reviews from critics. It was followed by a theatrical release in the United States on July 29, 1970, by Maron Films as Monster Zero, on a double bill with The War of the Gargantuas.

The film was followed by the seventh film in the Godzilla franchise, Ebirah, Horror of the Deep, released on December 17, 1966.