King Ghidorah (Japanese: キングギドラ, Hepburn: Kingu Gidora) is a giant dragon-like extraterrestrial monster, or daikaijū, in Toho’s Godzilla media franchise. It first appeared in the 1964 film Ghidorah, the Three-Headed Monster, directed by Ishirō Honda. The character was initially created by Tomoyuki Tanaka, Eiji Tsuburaya, and Shinichi Sekizawa as an homage to the eight-headed mythological Japanese dragon Yamata no Orochi. Although the name of the character is officially trademarked by Toho as "King Ghidorah", the character was initially referred to as Ghidrah in some English markets.

Although King Ghidorah's design has remained largely consistent throughout its appearances (an armless, bipedal, golden and yellowish-scaled dragon with three heads, two fan-shaped wings, and two tails), its origin story has varied from being an extraterrestrial planet-destroying dragon, a genetically engineered monster from the future, a guardian monster of ancient Japan, or a god from another dimension. The character is usually portrayed as the archenemy of Godzilla and a foe of Mothra, though it has had one appearance as an ally of the latter.

Despite rumors that Ghidorah was meant to represent the threat posed by China, which had at the time of the character's creation just developed nuclear weapons, director Ishirō Honda denied the connection and stated that Ghidorah was simply a modern take on the dragon Yamata no Orochi.

According to special effects director Eiji Tsuburaya's protégé Teruyoshi Nakano, the initial idea for Ghidorah, the Three-Headed Monster, came from Tomoyuki Tanaka, who also created Godzilla. Tanaka's inspiration came from an illustration of the Lernaean Hydra in a book about Greek mythology and Orochi of Japanese folklore. Tanaka was enamored with the idea of Godzilla fighting a multi-headed serpent but considered seven or eight heads excessive; thus, the number of heads was reduced to three. The final version, designed by Akira Watanabe, was a three-headed dragon with large wings, two tails, and of extraterrestrial origin.

Toho also drew inspiration from the three-headed dragon Zmey Gorynych or King Dragon キング・ドラゴン (Kingu Doragon) in the Japanese version of the 1956 Soviet film Ilya Muromets, which had been distributed theatrically in Japan by Shintoho in March 1959. King Ghidorah's name is composed of "King" (キング, Kingu) and "Ghidorah". The "Ghidorah" part of the name comes from the pronunciation of the word "hydra" (Гидра, ˈɡʲidrɐ) in Russian, written as ヒドラ (Hidora) in Japanese.

Other sources of inspiration included mythological creatures such as the hydra, cerberus, unicorn, pegasus, and qilin.

King Ghidorah primarily attacks by blasting "gravity beams" from its three mouths. These lightning-like attacks are roughly equal in power to Godzilla's atomic breath, and can be combined to form a stronger blast. King Ghidorah is seen using this ability in all its appearances except for Godzilla: The Planet Eater, where King Ghidorah's form is instead composed of energy resembling gravity beams. King Ghidorah has sometimes shown control over gravity in general, allowing the monster to fly even after losing a wing, crush and destroy thousands of planets (as well as spacefaring craft) sacrificed to it, and cause nearby objects in the environment to float due to its mere presence.

One trait that is seen in all versions of King Ghidorah is its great wings, which it uses to fly at Mach 3 (or 550 knots in Godzilla: King of the Monsters). In space, it can travel in a meteorite that produces magnetic fields, and fly at speeds exceeding Mach 400. Aside from this, it can use its wings to shield itself from enemy attacks (even Godzilla's atomic breath), project lightning strikes similar to its gravity beams, and generate supersonic shockwaves and strong winds. Ghidorah's electrical properties and generation of powerful bursts of wind cause yellow lightning storms to manifest in its vicinity. The resulting unnatural hurricane can tear through the stratosphere, raise tornadoes and waterspouts, and reach sustained wind speeds of 349 kilometres per hour (217 miles per hour), strengthening over time, breaking the Saffir–Simpson scale and exceeding the strength of any known storm on Earth. Although supplementary materials for Godzilla: King of the Monsters state that this happens when Ghidorah takes flight due to its hyper-tensile tendons, traces of gold in its scales, and electro-receptor molecular biology, in the film proper, the process starts as soon as Ghidorah awakens on the ground. As hinted by several scenes in the film and stated in the novelization, Ghidorah's hurricane furthermore disrupts and alters worldwide weather patterns to cause even more storm systems around the world. In the comic Kingdom Kong, which takes place in 2019, not even a year after Ghidorah's defeat by the end of King of the Monsters, the agency of Monarch reports that the storm has reached wind speeds of 644 kilometers (400 miles) per hour, with air pressure so dense that it triggers new cyclones every five minutes and creates waterspouts that reach down to the sea floor.