Gamera vs. Barugon (大怪獣決闘 ガメラ対バルゴン, Daikaijū kettō: Gamera tai Barugon, lit. 'Great Monster Duel: Gamera vs. Barugon') is a 1966 Japanese kaiju film directed by Shigeo Tanaka, with special effects by Noriaki Yuasa and Kazufumi Fujii. Produced by Daiei Film, it is the second entry in the Gamera franchise, and stars Kōjirō Hongō, Kyōko Enami, and Yūzō Hayakawa, with Teruo Aragaki as Gamera. In the film, Gamera returns to Earth to battle a reptilian monster born out of an opal brought to Japan by greedy entrepreneurs.

Due to the success of Gamera, the Giant Monster, studio president Masaichi Nagata pushed a sequel into production. Yunosaburo Saito was commissioned to write a story outline, which featured Gamera battling alien ice giants. Nagata instructed screenwriter Niisan Takahashi to write a "lavish" blockbuster and treat the monster spectacle seriously. Nagata promoted the project into an A-list production by approving a higher budget and attaching acclaimed talent. However, Yuasa, director of the previous film, was demoted to special effects director for this film. Principal photography for special effects began in January 1966 and ended in April 1966, while photography for drama scenes began in February 1966 and ended in April 1966.

Gamera vs. Barugon was theatrically released in Japan on April 17, 1966, on a double bill with Daimajin and underperformed at the Japanese box office. In 1967, it was released directly to television in the United States as War of the Monsters by American International Television. The film was followed by Gamera vs. Gyaos, released on March 15, 1967.