The official news agency KCNA reported the appointment of Hyon, who had been a general in the army, was made by the Central Military Commission of the Workers' Party of Korea and North Korea's National Defense Commission of the DPRK on Monday.
Illness was cited as the reason for Ri's removal but North Korea has not given any details. Ri had been close to the Communist country's new leader, Kim Jong Un, who took over after the death of his father Kim Jong Il in December. Ri also had been a member of the WPK Central Committee's Political Bureau and vice chairman of the WPK's Central Military Commission.
South Korea's Yonhap News Agency said the personnel changes had sparked media speculation in South Korea and elsewhere about the likelihood of a power struggle or other political changes in the isolated country.
Earlier, commenting on Ri's removal, one South Korean analyst told Yonhap the action might also have been because of Kim Jong Un's unsatisfactory military grip or as a result of a power struggle in North Korea. Ri had been seen as a guardian for the young Korean leader.
White House press secretary Jay Carney said he did not have any comment on the changes, saying the United States would judge North Korea's intentions based on its actions, Yonhap reported.
"And I think that the way we address the issue of policy towards North Korea has to do with holding North Korea accountable to its international obligations and judging North Korea by its actions, and not spending a lot of time trying to read into personnel moves in what is one of the world's most opaque governments and societies," Carney said.