Prince Hisahito and his elder sister, Princess Kako, attended a speech event for boys and girls in Tokyo on Sunday.
The two showed up at a public event together without being accompanied by any other imperial family member for the first time, according to the Imperial Household Agency.
They listened to the speeches of 12 junior high school students chosen from among 480,000 applicants.
The speakers talked about different topics, including peace. The top prize went to a boy who spoke about his experiences as a visually impaired person.
Prince Hisahito, 13, the son of Crown Prince Akishino and Crown Princess Kiko, is a first-grader at Ochanomizu University Junior High School. The teenager, the nephew of Emperor Naruhito, is second in line to the throne.
Princess Kako, 24, is the second daughter of the crown prince.