Klaatu was introduced in a short story by author Harry Bates in the October 1940 issue of Astounding Science Fiction.

In the film The Day the Earth Stood Still, Klaatu, portrayed by Michael Rennie, arrives in Washington, D.C. in a flying saucer alongside Gort, a robotic companion.

He asks Klaatu for a demonstration of power to show the potential threat the extraterrestrial alliance poses if Earth disrupts the peace in space.

In a gathering of scientists, Klaatu gives an ultimatum: Earth can join other spacefaring worlds, where peace is maintained through a powerful force of robots similar to Gort or face destruction.

The 2008 remake of The Day the Earth Stood Still, features Keanu Reeves as Klaatu and Jennifer Connelly as Helen Benson.

The two plan to destroy humanity to prevent the destruction of the planet, while preserving animal species in vessels similar to the concept of Noah's Ark.

Science fiction scholar Gary Westfahl writes that Klaatu's planet of origin in the 1951 film must be Mars, citing Klaatu's assertion that his journey covered 250 million miles (400 million kilometres)—the approximate distance between Earth and Mars when the planets are on diametrically opposite sides of the Sun.

[3] As corroborating in-story evidence, Westfahl points to Klaatu's line about being "neighbours" and a newspaper headline that reads "'Man from Mars' escapes from army hospital!".

[3][4] Moreover, Westfahl writes that portraying Martians as parental figures "is a ubiquitous trope in science fiction", and "It is a tradition that Klaatu fits into quite nicely.