About 150 workers had to leave the building on Tuesday morning as the fire took hold. No injuries are reported.
The hotel - which is famously where Princess Diana spent her last evening - is currently closed for renovations and there are no guests staying there.
The operation will take "a long time" as the fire is difficult to access, a fire service spokesman said.
The alert was raised at 07:00 local time (06:00 GMT) and about 60 firefighters and 15 fire engines were sent to the scene.
The blaze started on the seventh floor, took hold in the attic and spread to the roof.
Paris police tweeted that there was a "major fire", and advised motorists to avoid the Place Vendome area.
The area was blocked off at the height of the morning rush hour, while firefighters on cranes sprayed the flames.
Fire captain Yvon Bot said the fire was unusually difficult to access and control.
"There is no longer any risk of it spreading, but the operation will still take a very long time," he said.
"We are having to go metre by metre from the roof. It is too small to access from the interior."
He said a "large" section of the building was affected.
The hotel is a historic building in central Paris.
Princess Diana dined there on 30 August 1997. Later that night she and her companion Dodi al-Fayed were killed in a car crash. He was the son of the hotel's owner, Egyptian billionaire and Harrods owner Mohamed al-Fayed.
It was also the hotel of choice for Charlie Chaplin, Coco Chanel and Ernest Hemingway.
The Ritz was due to re-open in March after a three-year closure for multi-million euro renovations.
The Ritz name, used for hotels around the world, comes from a Swiss hotelier called Cesar Ritz, who set up the Paris Ritz and the London Ritz more than a century ago.
Both hotels are now independently owned. The Barclay brothers - newspaper owners Sir Frederick and Sir David Barclay - bought the London Ritz in 1995.