Japanese broadcaster NHK is reporting that Princess Akishino is pregnant, introducing the possibility of a male heir for the royal family.
NHK said the princess, known informally as Princess Kiko, expects to have her baby in September.
She is the wife of the crown prince's - Crown Prince Naruhito - younger brother Prince Akishino. The couple already has two girls.
The news comes as the country debates whether to allow women to inherit the imperial throne, since no males have been born into the royal family in 40 years.
At present, only male descendants of Japan's emperor can succeed him.
Some information for this report was provided by AP and Reuters.