Mechagodzilla (Japanese: メカゴジラ, Hepburn: Mekagojira) is a mecha kaiju (a giant robotic monster) who first appeared in the 1974 film, Godzilla vs. Mechagodzilla. In the Showa era Godzilla films, Mechagodzilla is depicted as being created by extraterrestrial invaders called Simeon to confront and destroy Godzilla. In the subsequent Heisei era, Millennium era and Monsterverse films, it is usually depicted as a man-made weapon designed to defend against Godzilla and other kaiju. The character is portrayed as a robotic counterpart of Godzilla with a vast array of high-tech weaponry. Along with King Ghidorah, Mechagodzilla is commonly considered to be an archenemy of Godzilla.

Mechagodzilla was conceived in 1974 as a more serious villain than its two immediate predecessors, Gigan and Megalon, whose films were considered creative disasters. Sources stated that Tomoyuki Tanaka, Jitsuzo Horiuchi and Yasuhiro Seto first came up with the idea for the character. with director Jun Fukuda and special effects director Teruyoshi Nakano agreeing on it. According to Tanaka, Mechagodzilla was inspired by both Mechani-Kong from the previous Toho film King Kong Escapes and the robot anime genre, which was popular at the time.

Effects director Teruyoshi Nakano also felt that a mechanical monster was cheaper to construct than the mutated animals Godzilla had previously faced. As the resulting Godzilla vs. Mechagodzilla proved to be a greater critical success than previous 1970s Godzilla films, the character was revived in 1975's Terror of Mechagodzilla. The film's screenplay was based on the winning entry of a story-writing competition won by Yukiko Takayama, who continued the darker tone of the previous film by adding the subplot of Mechagodzilla being cybernetically connected to a young woman. Mechagodzilla's design remained largely unchanged from its previous appearance, though it was made to look thinner and more angular, with a darker sheen and an MG2 insignia emblazoned on its upper arms. The film's original draft was going to have Mechagodzilla destroy Tokyo utterly, though the scale of destruction was cut down for budgetary reasons.

Mechagodzilla was brought back in 1993's Godzilla vs. Mechagodzilla II, as the success of Godzilla vs. Mothra and the popularity of its main antagonist Mothra spurred Toho into reintroducing familiar characters rather than inventing new ones. Originally, the new Mechagodzilla was to be named "Berserk", and was envisioned as being a much more organic Godzilla-like creature which would later turn on its creators after becoming infected with a computer virus which makes it self-aware. Berserk would subsequently absorb more and more machinery, to the point of degenerating into a mass of metal and wires, though this concept was rejected early in pre-production.

As Mechagodzilla was intended to be a military defense weapon rather than an alien construct, the character was redesigned as looking sleeker and smoother. It was portrayed by suit actor Wataru Fukuda, and consisted of multiple separate elements which were worn like plate armor. Special effects artist Koichi Kawakita originally envisioned Mechagodzilla being able to split into aerial and terrestrial units, though this idea was scrapped in favor of the character merging with the flying battleship Garuda. The film was promoted through the children's program Adventure Godzilla-land, which portrayed Godzilla and Mechagodzilla as rival news anchors reporting on the events of the upcoming movie. Composer Akira Ifukube wrote a theme for Mechagodzilla incorporating a slow battle march with heavy percussion and pentatonic phrasing.

The decision to incorporate Mechagodzilla into the Millennium series was taken by producer Shōgo Tomiyama, who gave the general outline of what would become the story of Godzilla Against Mechagodzilla to Godzilla vs. Megaguirus director Masaaki Tezuka. Tezuka instructed his staff to research both cybernetics and DNA engineering in order to make the character scientifically plausible. Tezuka had initially wanted Mechagodzilla to be a much speedier robot than the one on film, envisioning it as becoming progressively more agile during its fight against Godzilla as the latter tore off its opponent's armaments. This incarnation of Mechagodzilla was deliberately shown being airlifted by carriers rather than flying directly into battle as the previous two incarnations had done, as Tezuka felt that it made little sense for Mechagodzilla to drain its energy in such a manner. Upon being asked why the Mechagodzilla suit wasn't painted with military camouflage colors, Tezuka answered that "Mechagodzilla doesn't need to hide." Tezuka had originally intended to have both Godzilla and Mechagodzilla die at the end of the film, but was forced to change this on the insistence of Toho managers, who felt that such an ending was too dark for a New Year movie release.

In the 2003 sequel Godzilla: Tokyo S.O.S., creature designer Shinichi Wakasa used the same mold to create a redesigned Mechagodzilla, which was meant to look more weathered than its predecessor. According to production designer Shinki Nishikawa, several other changes were made to the Mechagodzilla design's head, chest and arms in order to make the character look less heroic and more machine-like. The robot's back unit was reduced in size and flipped upside down, and the head was made smaller. As the previous design's shoulder cannons were considered uneven and unsophisticated-looking because of their rectangular shape, special effects director Eiichi Asada made them more pentagonal. As the character was supposed to rely more on its forearm cannons than in the previous film, Nishikawa made them larger and more powerful looking.

For Godzilla vs. Kong, Mechagodzilla's design was inspired by the Generation 1 Transformers due to their "simplicity." Director Adam Wingard attempted to avoid the "complex" designs from the Michael Bay Transformers films. Writer Max Borenstein had originally written Mechagodzilla into Godzilla: King of the Monsters. However, director Michael Dougherty scrapped the character during development.