Police were using a Chinese translator to try to coax the man from a ledge on the outside of the glass wall overlooking the building's atrium, The Hill newspaper reported.
"We don't know very much right now," said Senate Sergeant at Arms Terrance Gainer, though he added police had previous contact with the man. "We don't know what his issue is or what office he visited. The police are trying to calm the situation and see if we can get him on the right side of the rail."
Emergency workers were erecting safety equipment should he decide to leap, Gainer said.
"Hopefully it won't come to that," he said.
While the atrium was closed off, the building was not evacuated, the newspaper said. The seventh floor of the Hart building holds the offices of 14 senators, including Democratic presidential candidate Sen. Barack Obama, D-Ill.