The Imperial Household Agency announced that Prince Mikasa had died in the morning at a hospital in Tokyo.
Japanese media reports say he had been hospitalised since May, initially because of pneumonia. Citing unnamed sources, they say his condition took a sudden turn for the worse today.
Prince Mikasa was born on December 2, 1915. He is the uncle of the current Emperor Akihito and was fifth in line to the throne.
His brother Hirohito reigned for more than 60 years, during World War II and until his death in 1989.