The third son of North Korea's leader, believed to be no older than 28, was promoted to the rank of four-star general last week, South Korea's Yonhap news agency reported.
The promotion signaled his selection as heir apparent to his father and the next leader of the North.
Also at the army drill was Kim's 64-year-old sister, Kim Kyong Hui, who was promoted to a full general, a report from official North Korean media said.
The report did not say when or where the live-fire drill took place.
Kim Jong Un became a vice chairman of the Central Military Commission of the Workers' Party last week in a move that signaled the start of another father-to-son succession of power.