The noose was hanging from a lamp post Saturday outside the Washington museum, US Park Police Sgt. Anna Rose said.
It's the third time this symbol of racial violence has turned up in the National Mall area in recent weeks.
Last month, two other nooses were found at the
National Museum of African American History and Culture on May 31, and the Hirshhorn Museum on May 26.
In other incidents away from the National Mall, a noose was found at a construction site in Washington this month, police said.
Officers went to a house under construction in southeast Washington and "discovered a rope, tied in a noose, displayed by the front door," a police report said. The property manager said the noose was not present when work ended the previous day.
At in April, a noose was left in the kitchen of the Phi Kappa Tau fraternity house
at the University of Maryland.
Washington Mayor Muriel Bowser has condemned the incidents in his city.
"We are an inclusive city, and we do not tolerate signs of hate, ignorance and fear," she said this month. "Our diversity is what makes us stronger, and we will not relent in promoting and defending DC Values. We do not take these incidents lightly, and we will not accept that signs of hate are signs of our time."