Ronald William Prestige, 59, of Camden, S.C., was arrested around 9:20 a.m. attempting to enter the Cannon Office Building while carrying a 9mm Ruger handgun in his bag.
Prestige was handcuffed and escorted out of the building by two police officers, Capitol Police said. He is not a Hill staffer, and faces charges of carrying a pistol without a license.
The rotunda entrance of Cannon was closed for about a half an hour, and reopened shortly before 10 a.m.
Last Friday, press secretary Ryan Shucard was arrested for attempting to enter the Cannon building while carrying a 9mm Smith and Wesson handgun and magazine.
Shucard, who works for Rep. Tom Marino, R-Pa., was placed on unpaid leave and on Saturday pleaded not guilty to felony charges of carrying a pistol.
All people entering buildings on the Capitol Hill complex are required to pass through metal detectors and send bags through an x-ray belt, although members of Congress are generally allowed to pass through without a check.