Prince Tomohito, a cousin of Emperor Akihito, has died at the age of 66, Japan's imperial palace said today.
The prince died in hospital in Tokyo.
The Imperial Household Agency did not give a cause for his death, but he had been suffering from several illnesses, including throat cancer.
He had several cancer-related operations over the last two decades, and was treated for alcoholism in 2007.
Prince Tomohito was the eldest son of Prince Mikasa and Princess Yuriko.
He is survived by his wife and two children.