The changes, which went into effect Wednesday at 11 p.m., will prevent the public from accessing the sidewalk, roadways and the grounds between the south fence line and E Street NW between West Executive Avenue and East Executive Avenue, the agency said.
"Restricting public access to the fence line will not only serve to lessen the possibility of individuals illegally accessing the White House grounds", said Milhoan, "but will also create a clear visual break to enable Secret Service officers to identify and respond to potential hazards including individuals attempting to scale the fence".
The U.S. Secret Service has similar restrictions on the northern fence at the White House, the agency told Reuters.
The stretch of pavement along the south fence has been closed each night - between 11pm and 6am - since 2015.
The move comes after a series of recent efforts to shield President Donald Trump from public scrutiny. More highly symbolic First Amendment space is gone.
On March 10, Jonathan T. Tran, 26, jumped three fences to gain access the south entrance of the White House with two cans of mace, a backpack and a letter for the president.
The new restriction won't require building any new physical barriers, Milhoan said.More news: Adidas sorry for email saying 'you survived' Boston Marathon
Up until May 1995, traffic plied on Pennsylvania Avenue right in front of the White House, a road which once saw tramcars - and horse drawn carriages before that.
There's a plan for a significantly taller fence with "pencil point" anti-climb fixtures, but construction isn't slated to start until next year.
Passers-by will still be able to see and take photographs of the White House and its grounds.
Secret Service Communications Director Cathy Milhoan told local radio station WTOP that the closure was not in response to any specific threat.
The same restrictions are in place on the north fence of the White House grounds, according to the Secret Service.
The sidewalk is popular among tourists because of the iconic view of the White House.