The Lone gunman theory (a.k.a. the "lone nut theory") is the nickname given to the conclusion reached by the Warren Commission that U.S. President John F. Kennedy was assassinated by a single gunman named Lee Harvey Oswald who fired only three shots, one of which being the single bullet that wounded both Kennedy and Governor John Connally. The Commission report stated that Oswald was a disturbed man, whose radical political views and depression had led him to shoot the President.

In the late 1970s, the House Select Committee on Assassinations concluded that President Kennedy was "most likely killed as the result of a conspiracy", though this conclusion is disputed because of its reliance on audio evidence which is believed to be poorly interpreted.

As a plot device on the FOX Network's popular conspiracy drama The X-Files, protagonist Fox Mulder often solicited the aid of a small group of independent, underground intelligence specialists who chose for their name the ironically pluralized "lone gunmen". "The Lone Gunmen" was spun-off in 2001, documenting the group's activities independent of the X-Files, but was cancelled in its first season.