Police are investigating after a noose was found in an exhibit on racial segregation at the National Museum of African American History and Culture.
Museum director Lonnie Bunch said it “has long represented a deplorable act of cowardice and depravity”.
It is the second noose found at a Smithsonian museum in four days.
The looped rope is synonymous with the extrajudicial hangings, or lynchings, of mainly African-American people in the late 19th and early 20th Centuries.
Another noose was found hanging from a tree in the Hirshhorn Museum and Sculpture Garden on Friday, steps away from the National Mall footpath which legions of tourists pass by each day.
US Park Police are investigating the incidents.
“Today’s incident is a painful reminder of the challenges that African Americans continue to face”, Mr Bunch wrote in an email to staff.
“We will continue to help breach the chasm of race that has divided this nation since its inception,” he added.