The museum, on the National Mall near the White House, reopened fully around 3:15 p.m., after police deemed the area "safe and secure," according to an internal memo provided to ABC News by the Smithsonian.
In an article posted to the Smithsonian site Wednesday evening, museum officials said the noose is a reminder of "America's dark history with lynching" and referred to similar incidents of noose sightings around the country including at a school in Missouri, a construction site in Maryland, at Duke University campus, at a fraternity house on University of Maryland campus, at a middle school in Maryland and at a high school in Lakewood, California.
The museum, which opened late last year, said the "horrible act" showed just how necessary its mission is.
"Today's incident is a painful reminder of the challenges that African-Americans continue to face," the museum's director, Lonnie Bunch, wrote in a memo to employees on Wednesday. "This was a horrible act, but it is a stark reminder of why our work is so important."
The rope was found in the museum's history gallery, which features an exhibit on segregation.
Visitor Lori Nzekwe of Tampa, Florida, told ABC affiliate WJLA that she and her 6-year-old daughters saw the blocked-off area where authorities were investigating.
"It's shocking that in 2017 we still have this type of bigotry," Nzekwe said.
The U.S. Park Police and the Smithsonian Office of Protection Services are currently investigating the incident.
It was the second time a noose was found on Smithsonian property in D.C. in the last week.
Last Friday, police conducting a security check found a noose hanging on a tree outside the Smithsonian's Hirshhorn Museum, authorities said.
It was not clear if the two incidents were related.
Data from the Southern Poverty Law Center shows that more than 1,300 hate-related incidents have been reported between the 2016 election and February 2017. Of those reported incidents, anti-immigrant cases have been the most prevalent, followed by anti-black cases according to the data.
“We haven’t seen such mainstream support for hate in decades, not since the Civil Rights era 50 years ago,” the Smithsonian reports SPLC spokesperson Ryan Lenz saying. “We’re witnessing a moment when there are tremendous challenges to the country that we built on pluralism and democracy. The civil rights accorded every American are firmly under threat.”
In his email memo to staff, Bunch assured that "we will continue to help reach the chasm of race that has divided this nation since its inception."
The museum thanked the public for their support in a tweet Thursday.
ABC News Courtney Connley contributed to this report.