Police said the man had a "social justice" protest sign with him when he produced a weapon and shot himself on the western side of the building in the afternoon.
US Capitol Police Chief Kim Dine said no shots were fired by police. His force investigated a backpack and case the man had with him before reopening the legislative building and its visitor center.
Dine said the shooting was not terrorism-related, declining to comment further on the man's identity or his motive pending an investigation.
The FBI, Secret Service and local police are investigating the shooting.
"We potentially know who he is but obviously we have to secure that and verify it," Dine said.
View galleryA member of the bomb squad responds to reports of a …
A member of the bomb squad responds to reports of a shooting at the US Capitol in Washington, DC, Ap …
Washington is in the middle of one of its peak tourism seasons as visitors flock to view the city's famous cherry blossoms.