The War of the Gargantuas, released in Japan as Frankenstein's Monsters: Sanda versus Gaira (フランケンシュタインの怪獣 サンダ対ガイラ Furankenshutain no Kaijū: Sanda tai Gaira?), is a 1966 science fiction kaiju film. The film was co-produced between the Japanese company Toho, and Henry G. Saperstein's American company UPA. The film was a sequel to Frankenstein Conquers the World, and was the second of two films featuring giant Frankenstein monsters that Sapertstein's company co-produced with Toho. Directed by Ishirō Honda, and featuring special effects by Eiji Tsuburaya, the film starred Hollywood actor Russ Tamblyn, alongside Japanese actors Kenji Sahara and Kumi Mizuno.

The film tells the story of two giant, hairy humanoids called Frankensteins (Gargantuas in the American version) that were spawned from the discarded cells of Frankenstein's monster from the previous film. A green one raised in the sea named Gaira (ガイラ?, fromkai, "sea") is violent and savage, while a brown one who resides in the Japan Alps, named Sanda (サンダ?, from san, "mountain") is friendly and docile. The film follows the investigation and military engagements of these creatures until their climactic confrontation in Tokyo.

The film was released theatrically in the United States in the summer of 1970 by Maron Films where it played nationwide on a double bill with Monster Zero.