People in the area are being evacuated, said a police spokesman, although it is unclear how many evacuees there are or how big the fire is.
The flames started near the main UNESCO World Heritage Hall, according to the Naha City Fire Department, Okinawa Prefecture, Japan's national television broadcaster NHK.
Shuri Castle has been the seat of the Ryukyu Kings for over 400 years.
The castle was completely destroyed during the Battle of Okinawa in 1945, and the restoration work was only completed in the early 1990s.
The entire region was significantly damaged during World War II when Shuri Castle acted as the local headquarters of the Imperial Japanese Army.