The percentage is the lowest in the nearly 50 years since the killing, Gallup reported.
In 1976, 81 percent of Americans said they believe in a conspiracy theory, despite the Warren Commission investigation that ruled that Lee Harvey Oswald acted alone.
The belief in the conspiracy theory remained high for many subsequent polls, Gallup said.
In the most recent poll, Americans said they think the Mafia, federal government, CIA and/or Fidel Castro could have been involved.
Thirty percent of respondents said Oswald acted alone.
The survey queried 1,039 adults by phone Nov. 7-10. The margin of error was 5 percentage points.