The announcement came in the wake of the removal army chief Ri Yong Ho, who had been a close confidant of Kim, and the promotion of Gen. Hyun Yong Chol to vice marshal.
The latest announcement was carried by the official Korean Central News Agency, China's state-run Xinhua News Agency reported.
"A decision was made to award the title of marshal of the DPRK [North Korea] to Kim Jong Un, supreme commander of the Korean People's Army," the report said. "The decision was jointly issued on Tuesday by the Central Committee and the Central Military Commission of the Workers' Party of Korea, the National Defense Commission of the DPRK and the Presidium of the Supreme People's Assembly of the DPRK."
North Korea has a 1.2-million-strong army.
South Korea's Yonhap News Agency said the removal of Ri is being interpreted by senior officials in Seoul as the first political purge by Kim Jong Un as he consolidates his power. He became leader in December after the death of his father Kim Jong Il.
"Kim Jong Un was given the title he deserves as the firmly seated North Korean leader and the supreme commander," Paik Hak-soon, senior researcher at the Sejong Institute, told Yonhap. "The title awarding came at a time Kim is solidifying his power."