The Emperor of Nihon-Ja is the tenth installment of the Ranger's Apprentice book series by Australian author John Flanagan. The book was released in Australia on 1 November 2010, in New Zealand on 5 November 2010, and in the United States on 16 April 2011. The name Nihon-Ja is based on the name of Japan, Nihon-Koku.

John Anthony Flanagan is an Australian fantasy author best known for his medieval fantasy series, the Ranger's Apprentice series, and its sister series, the Brotherband Chronicles. Some of his other works include his Storm Peak duology, as well as the adult novel The Grey Raider.