Kaiju (Japanese: 怪獣（かいじゅう）, Hepburn: kaijū; lit. 'strange beast'; Japanese pronunciation: [kai(d)ʑɯː]) is a Japanese term that is commonly associated with media involving giant monsters. A subgenre of science-fiction, more precisely monster films, its widespread contemporary use is credited to tokusatsu (special effects) director Eiji Tsuburaya and filmmaker Ishirō Honda, who popularized the kaiju film genre by creating the Godzilla franchise and its spin-offs. The term can also refer to the monsters themselves, which are usually depicted attacking major cities and battling either the military or other creatures.

Godzilla (1954) is often regarded as the first kaiju movie. When developing it, Honda and Tsuburaya drew inspiration from the character of King Kong, both in its influential 1933 film and in the conception of a giant monster, establishing it as a pivotal precursor in the evolution of the genre. During their formative years, kaiju movies were generally neglected by Japanese critics, who regarded them as "juvenile gimmick", according to authors Steve Ryfle and Ed Godziszewski.

Kaiju are often somewhat metaphorical in nature; Godzilla, for example, initially served as a metaphor for nuclear weapons, reflecting the fears of post-war Japan following the atomic bombings of Hiroshima and Nagasaki and the Lucky Dragon 5 incident. Other notable examples of kaiju include Rodan, Mothra, King Ghidorah, Gamera, and King Kong.

The Japanese word kaijū originally referred to monsters and creatures from ancient Japanese legends; it earlier appeared in the Chinese Classic of Mountains and Seas. There are no traditional depictions of kaijū or kaijū-like creatures among the yōkai of Japanese folklore, although it is possible to find megafauna in their mythology (e.g., Japanese dragons). After sakoku ended and Japan was opened to foreign relations in the mid-19th century, the term kaijū came to be used to express concepts from paleontology and legendary creatures from around the world. For example, the extinct Ceratosaurus-like cryptid featured in The Monster of "Partridge Creek" (1908) by French writer Georges Dupuy was referred to as kaijū. It is worthy to note that in the Meiji era, Jules Verne’s works were introduced to the Japanese public, achieving great success around 1890.

Genre elements were present at the end of Winsor McCay's 1921 animated short The Pet in which a mysterious giant animal starts destroying the city, until it is countered by a massive airstrike. It was based on a 1905 episode of McCay's comic strip series Dreams of the Rarebit Fiend.

The 1925 film The Lost World (adapted from Arthur Conan Doyle's 1912 novel of the same name), featured many dinosaurs, including a brontosaurus that breaks loose in London and destroys Tower Bridge. The dinosaurs of The Lost World were animated by pioneering stop motion techniques by Willis H. O'Brien, who would some years later animate the giant gorilla-like creature breaking loose in New York City in the 1933 film King Kong. The enormous success of King Kong can be seen as the definitive breakthrough of monster movies. This influential achievement of King Kong paved the way for the emergence of the giant monster genre, serving as a blueprint for future kaiju productions. Its success reverberated in the film industry, leaving a lasting impact and solidifying the figure of the giant monster as an essential component in genre cinematography. RKO Pictures later licensed the King Kong character to the Japanese studio Toho, resulting in the co-productions King Kong vs. Godzilla (1962) and King Kong Escapes (1967), both directed by Ishirō Honda.

Yoshirō Edamasa directed The Great Buddha Arrival in 1934. Although the original film is now lost, stills of the film have survived, and it is one of the earliest examples of a kaiju film in Japanese cinematic history. The 1934 film presumably influenced the production of the Ultraman franchise.

Ray Bradbury's short story published in the Saturday Evening Post, June 23, 1951 "The Beast from 20,000 Fathoms" served as the basis for the film The Beast from 20,000 Fathoms (1953), featuring a fictional dinosaur (animated by Ray Harryhausen), which is released from its frozen, hibernating state by an atomic bomb test within the Arctic Circle. The American movie was released in Japan in 1954 under the title The Atomic Kaiju Appears, marking the first use of the genre's name in a film title. However, Godzilla, released in 1954, is commonly regarded as the first Japanese kaiju film. Tomoyuki Tanaka, a producer for Toho Studios in Tokyo, needed a film to release after his previous project was halted. Seeing how well the Hollywood giant monster movie genre films King Kong and The Beast from 20,000 Fathoms had done in Japanese box offices, and himself a fan of these films, he set out to make a new movie based on them and created Godzilla. Tanaka aimed to combine Hollywood giant monster movies with the re-emerged Japanese fears of atomic weapons that arose from the Daigo Fukuryū Maru fishing boat incident; and so he put a team together and created the concept of a giant radioactive creature emerging from the depths of the ocean, a creature that would become the monster Godzilla. Godzilla initially had commercial success in Japan, inspiring other kaiju movies.