North Korea's state media says the country's army chief Ri Yong-ho has been removed from all posts "because of illness."
Mr Ri was made army chief three years ago under Kim Jong-il, the current leader's father who died in December 2011 after ruling North Korea for almost two decades.
The 70-year-old was known as key figure and confidant who helped new leader Kim Jong Un seize military control
Seoul's unification ministry said it could not confirm why and how the decision was made, but it is unusual that North Korea announced the move so quickly.
Spokesman Kim Hyung-suk said: "It's quite a rare case that the North promptly and publicly announced early this morning the outcome of a meeting yesterday, July 15. We will keep closely monitoring."
Mr Ri's removal is now being scrutinised by analysts, who are looking for more clues about the direction in which Kim Jong-un will take his country.