North Korean military chief Ri Yong-ho has been removed from all official posts, according to state media.
As well as being head of the army, he was vice-chairman of the powerful Central Military Commission and held top posts in the ruling Workers Party.
In a short statement, the party said Mr Ri had been removed from his posts "because of illness".
The BBC's Seoul correspondent Lucy Williamson says there is widespread scepticism about that explanation.
The decision to remove Mr Ri from power is now being scrutinised by analysts for signs about which direction North Korea's new leader Kim Jong-un will take the country in.
He was made army chief three years ago under the rule of Kim Jong-il, the current leader's father.
Mr Ri regularly appeared at state occasions beside Kim Jong-il and was seen as a key figure in the transition of power to his son.
Few details are known about Kim Jong-un, who has now in power for six months, but he is believed to be less than 30 years old.
Our correspondent says control of North Korea's military is seen as key to retaining power, but Kim Jong-un's more relaxed leadership style has led some to predict wider signs of change within the closed Stalinist regime.