Internal Security Forces detained a man in a suburb of Beirut, according to a statement posted to Facebook. The weapon thought to have been used in the attack was also seized, The National News reported.
The Lebanese man, born in 1997, was only referred to by the initials MK. He admitted to the shooting, police said.
Shots were fired near the US embassy in the northern suburb of Awkar on Wednesday, the anniversary of a 1984 bombing that killed 24 people at the diplomatic mission.
No one was injured in the shooting. An investigation is now underway, the ISF added.
No one claimed responsibility for the shooting and the motives behind it were not known.
American forces and diplomats have come under attack several times in Lebanon, prompting the US to close its embassy and withdraw all diplomatic staff between 1989 and 1991.