People in the area are being evacuated, said a police spokesperson, though it is unclear how many evacuees there are or how big the fire is.
The blaze started near the main hall of the UNESCO World Heritage Site, according to the fire department in Naha City, Okinawa Prefecture, Japan’s national broadcaster NHK reported.
Shuri Castle was the seat of the kings of Ryukyu for more than 400 years.
The castle was completely destroyed during the Battle of Okinawa in 1945, and reconstruction work was only completed in the early 1990s.
The entire region was considerably damaged during World War II, when Shuri Castle acted as the local headquarters for the Imperial Japanese Army.