Majestic 12, also known as Majic-12, and MJ-12 for short, is a purported organization that appears in UFO conspiracy theories. The organization is claimed to be the code name of an alleged secret committee of scientists, military leaders, and government officials, formed in 1947 by an executive order by U.S. President Harry S. Truman to facilitate recovery and investigation of alien spacecraft. The concept originated in a series of supposedly leaked secret government documents first circulated by ufologists in 1984. Upon examination, the Federal Bureau of Investigation (FBI) declared the documents to be "completely bogus", and many ufologists consider them to be an elaborate hoax. Majestic 12 remains popular among some UFO conspiracy theorists and the concept has appeared in popular culture including television, film and literature.

UFO conspiracy theories are a subset of conspiracy theories which argue that various governments and politicians globally, in particular the United States government, are suppressing evidence that unidentified flying objects are controlled by a non-human intelligence or built using alien technology. Such conspiracy theories usually argue that Earth governments are in communication or cooperation with extraterrestrial visitors despite public disclaimers, and further that some of these theories claim that the governments are explicitly allowing alien abduction.