Gappa: The Triphibian Monster (Japanese: 大巨獣ガッパ, Hepburn: Daikyojū Gappa; lit. 'Giant Beast Gappa') is a 1967 Japanese kaiju film directed by Haruyasu Noguchi. The film is about a group of Japanese reporters who discover an infant monster called a Gappa on Obelisk Island. The reporters cage the creature and take it to Japan where it becomes a media attraction. This angers the natives of the island and Gappa's full-grown parents, who head toward Japan to find their child. Its plot virtually duplicates that of the 1961 British film Gorgo.

The film was released theatrically as Daikyojū Gappa in Japan in 1967, but only received a direct-to-television release in the United States as Monster from a Prehistoric Planet. It received positive reviews from Variety and Phil Hardy.

Certain shots of the Gappas attacking Japan were used in the 1991 Red Dwarf episode "Meltdown".

An expedition from Tokyo heads to Obelisk Island, which the greedy Mr. Funazu, owner of Playmate Magazine, wants to turn into a resort. The island natives welcome the expedition, but two members, Hiroshi and Itoko, venture into a forbidden area despite the pleas of a native boy named Saki. They enter a cavern blocked by a fallen statue and find a giant egg, out of which hatches a baby "bird-lizard" monster, referred to as "Gappa". The natives plead with the skeptical scientists not to take the baby away, lest it anger the baby's parents, but they do so anyway. Inside the caverns, Gappa's two parents rise from the subterranean waters beneath the volcano, destroying everything in their path. Saki, the only survivor, is rescued by an American Navy fleet and brought to Japan.

Meanwhile, Gappa makes global headlines and is experimented on by scientists. To the shock of the expedition members, two giant flying creatures appear over Sagami Bay. The Gappa parents ravage cities looking for their offspring and prove impervious to military weapons. Hiroshi, Itoko, Saki, and expedition scientist Professor Tonooka try to persuade the headstrong Mr. Funazu to let the baby go and return it to its parents. Mr. Funazu finally lets Gappa go back to its parents. Then the three go back to Obelisk Island.

The year 1967 was the height of Japan's “Kaiju Boom” and many Japanese film studios were doing their own monster film. Nikkatsu jumped onboard and decided to do their own film. Several ideas were hatched but not bore fruit. there was Giant Monster Gigant (大怪獣ギガント, Daikaiju Giganto): A giant alien lifeform resembling a spider arrives on Earth and causes destruction. Giant Squid Monster Arkitius (大烏賊アーキティウス, Oiki Akitiusu): During World War II, a Nazi U-boat is attacked by a giant squid called Arkitius. Giant Monster Momonra (怪獣モモンラ, Daikaiju Momonra): Japan is attacked by a giant mutated flying squirrel. And Reigon: Devil of the Seabed (海底の魔王レイゴン, Kaitei no mao Reigon): A giant manta ray dubbed “Reigon” appears one day and wreaks havoc across the world and then fights a giant iguana. This was the fourth and final unmade film in a series of concepts Nikkatsu Company had before making Gappa. This concept got the furthest into development out of the four scrapped projects, receiving a plot synopsis, speculative screenplay, and kaiju ideas. Planning was done by Hideo Kodama; the draft was written by Shunichi Yukimuro and Ryuzo Nakanishi.

In an interview with the film's screenplay writers, Gen Yamazaki and Ryuzo Nakanishi, they explained that the film got government financing of about 500,000,000 yen (about $1.4 million), which was ten times the average of a Nikkatsu film. Eisei Koi, who was the producer of the film was classmates with politicians in the Diet and used his political power to get the money.

Principal photography on Gappa lasted for about 40 days, twice the time that director Haruyasu Noguchi usually took to shoot a film.