The White House has returned to normal operations after being placed on lockdown, the Secret Service said.
Secret Service spokesman Robert Hoback said someone threw a metal object over the north fence along Pennsylvania Avenue and was arrested without incident.
He did not identify the person, though a witness saw a woman placed in handcuffs.
Fire engines and a hazardous materials response unit were called to the scene. It is common for officers to sweep the grounds after such incidents to ensure nothing dangerous was thrown over the fence.
Mr Hoback said the sweep results were negative.
The lockdown ensued shortly after President Barack Obama returned from a Memorial Day observance at Arlington National Cemetery.