The woman was spotted by Secret Service agents around 8:30 p.m. walking along the north fence on Pennsylvania Avenue.
Secret Service special agents spotted a gun in a holster around the woman’s waist and then arrested the woman.
Officials say the woman was carrying a 9mm handgun in the holster. She is identified as 23-year-old April Lenhart of Michigan.
She is reportedly undergoing further questioning, but is said to be uncooperative.
President Obama was in the White House at the time of the arrest. He had finished his Address to the Nation from the East Room 15 minutes earlier.
This comes a day after a man was arrested near the White House after approaching a checkpoint claiming Obama had invited him. Agents later found a .30-30 rifle, ammunition and a knife in his car.