Brad Pitt said during the 2012 Academy Awards that this was the first movie he ever saw.

During the process of dubbing the film for American audiences, Russ Tamblyn's audio track was lost, meaning he had to re'dub all his lines, which might explain why he sounds so disinterested throughout the movie.

This film was a sequel to the film Furankenshutain tai chitei kaijû Baragon (1965). However, all references to the previous film were cut from the American version of this film.

In one draft of the script, the film was to end with the volcano not only destroying Sanda and Gaira, but the lava was to destroy Tokyo and the remaining cells of the two monsters as well.

Haruo Nakajima, the man who played many of the Toho monsters (including Godzilla in 11 of the original 15 movies) has said Gaira, the evil Gargantua was his favorite to play due to the flexibility of the costume and because his eyes were visible through the mask, allowing him to emote better.

The part of the American singer was played by Kipp Hamilton, sister- in- law of Carol Burnett.

The opening scene features a giant octopus originally created for the never-used alternate ending that was shot for the American release of Furankenshutain tai chitei kaijû Baragon (1965).

Russ Tamblyn reportedly hated working on the movie and often disobeyed the film crew's orders, causing much frustration to his peers. He considered the original script dreadful and rewrote or improvised a lot of his dialogue, at times confounding his Japanese interpreter. Nevertheless, he has warmed up to the film decades later and now claims that he likes it.

One of Nicolas Cage's favorite films.

Tab Hunter was originally chosen to play Dr. Paul Stewart.

Quentin Tarantino has cited this movie as an influence for his film career.

This was doubled with Monster Zero which starred Nick Adams who was in Frankenstein Conquers the World; which War of the Gargantuans was a sequel to.

Even though both Sanda and Gaira died at the end of the film, there was talk of one or both of them returning to "star" in a film with Godzilla. Unfortunately, those plans never made it past the talking stages.