Majestic 12, also known as MJ 12, is believed to be the codename of a covert group of scientists, military leaders and government officials that was created in 1947 under the executive orders of President Harry S. Truman. Created with the function of investigating UFO activities after the incident in Roswell, Majestic 12 consisted of six civilians – mostly scientists – and six high-ranking military officers, two from each of the three major military services.

The only evidence that Majestic 12 existed was a collection of documents that surfaced in 1984 that stated the date of the creation of the group. The United States government has denied the existence of Majestic 12, insisting that the papers were a hoax. The UFO community were divided by the documents, one half insisting the documents were genuine while the other agreed with the FBI's assertion of it being a hoax due to the errors in both the formatting and the chronology. Since the first Majestic 12 papers, thousands of papers have appeared and collectively been called the "Majestic Documents." All of these documents are allegedly dated from 1942 to 1997, ranging from contact with aliens to testing of alien spacecraft to cover-up memos. One Majestic 12 document that is a considered a manual dated in 1954 is called the MJ-12 "Special Orders Manual (SOM)," which mainly dealt with the handling of crash debris and alien bodies. The genuineness of this manual has been debated, centering on the style of how it was written and the historical anachronisms. The Majestic Documents also supposedly contained the signatures of important people such as Albert Einstein and Ronald Reagan, sparking heated debates in the UFO and conspiracy groups. The authenticity of these papers has been highly questioned with only a few proven to be fraudulent. No other Majestic 12 documents have been released since 1997.

Majestic 12 has been associated with NSC 5412/2 Special Group created by President Eisenhower in 1954, an extremely secretive group that has been historically verified. While the Special Group had nothing to do with UFOs, the "12" has been noted between the two groups as well as the first chairman of the Special Group, Gordon Gray, who had been believed to be a member of Majestic 12. Majestic 12 was also believed to go under the more public name of Interplanetary Phenomenon Unit which was a UFO-related military group active between the 1940s and 1950s.

In present-day Majestic 12 is believed to just cover up alien activities on Earth, and trade with the alien for advance technology in return for knowledge and testing of human biology. The group has also been referenced in today's pop-culture such as the television show Dark Skies, and the video games Deus Ex and its sequel Deus Ex: Invisible War.