The incident occurred just before midnight on Friday while President Donald Trump was inside.
The intruder, named as Jonathan Tran, was spotted by a Secret Service agent by the south side of the executive residence, according to an official quoted by CNN.
The president, who is spending the weekend in Washington, was alerted late on Friday.
Officials say he was not in danger, although the White House was reportedly placed on an “orange” alert – one of the highest security levels used by the Secret Service.
In a statement to US media, the Secret Service said the backpack was screened and searched – with no hazardous material found.
On Saturday President Trump praised the Secret Service for doing a “fantastic job”.
He said the intruder was a “troubled person”.
It is not the first breach of White House security in recent years.
In 2014 a man made it past eight Secret Service agents with a three-and-a-half-inch folding knife in his trouser pocket.
A report found that a breakdown in radio communications helped Omar Gonzalez, from Texas, make it all the way to the East Room before being apprehended.
The Obamas were not inside at the time.
Officers later recovered more than 800 rounds of ammunition, a machete and two hatchets from the 42-year-old’s car.
Gonzalez was sentenced to 17 months in prison.
On Thanksgiving in 2015 the Secret Service apprehended Joseph Caputo, from Connecticut, on the North Lawn.
He had scaled the wall wearing an American flag cape while the first family was inside.
He was sentenced to three years on probation.