The War of the Gargantuas (Japanese: フランケンシュタインの怪獣 サンダ対ガイラ, Hepburn: Furankenshutain no Kaijū: Sanda tai Gaira, lit. 'Frankenstein's Monsters: Sanda vs. Gaira') is a 1966 kaiju film directed by Ishirō Honda, with special effects by Eiji Tsuburaya. Referred by film historian Stuart Galbraith IV as a "quasi–sequel" to Frankenstein vs. Baragon, the film was a Japanese-American co-production; it was the third and final collaboration between Toho Co., Ltd and Henry G. Saperstein. The film stars Russ Tamblyn, Kumi Mizuno and Kenji Sahara, with Yû Sekida as Sanda and Haruo Nakajima as Gaira. In the film, scientists investigate the sudden appearance of two giant hairy humanoid monsters that culminates in a battle in Tokyo.

The script's final draft was submitted in April 1966. Tab Hunter was originally hired by Saperstein, choosing to replace Nick Adams. However, Hunter was replaced by Tamblyn during pre-production. Honda's contract was not renewed and had to seek employment by speaking to Tanaka on a film-by-film basis. Honda, Saperstein, and chief assistant Seiji Tani noted that Tamblyn was difficult to work with. Tamblyn did the opposite of Honda's instructions, and improvised his lines without Honda's approval. Principal photography began in May 1966 and wrapped in June 1966, with effects photography concluding in July 1966.

The War of the Gargantuas was theatrically released in Japan on July 31, 1966, followed by a theatrical release in the United States on July 29, 1970 on a double feature with Monster Zero. Since its release, the film has been regarded as a cult classic, drawing admiration from artists such as Brad Pitt, Quentin Tarantino, Guillermo del Toro and Tim Burton.