Klaatu is a fictional humanoid alien character best known from his appearances in the 1951 science fiction film The Day the Earth Stood Still and its 2008 remake. The character of Klaatu gained popularity partly due to the iconic phrase "Klaatu barada nikto!" associated with the character.

The Day the Earth Stood Still is a 1951 American science fiction film from 20th Century Fox, produced by Julian Blaustein and directed by Robert Wise. It stars Michael Rennie, Patricia Neal, Hugh Marlowe, Sam Jaffe, Billy Gray, Frances Bavier and Lock Martin. The screenplay was written by Edmund H. North, based on the 1940 science fiction short story "Farewell to the Master" by Harry Bates. The film score was composed by Bernard Herrmann. Set in the Cold War during the early stages of the nuclear arms race, the storyline involves a humanoid alien visitor who comes to Earth, accompanied by a powerful robot, to deliver an important message that will affect the entire human race. In 1995, the film was selected for preservation in the United States National Film Registry as "culturally, historically, or aesthetically significant".