Kim Jong Un used a meeting of his Workers' Party to reveal he is promoting his younger sister.
The 33-year-old told the party's Central Committee that Kim Yo Jong, 28, would become an alternate member of the politburo, the country's top decision-making body of which Mr Kim is president.
His sister's promotion 'shows that her portfolio and writ is far more substantive than previously believed', said Michael Madden, a North Korea expert at Johns Hopkins University's 38 North website.
He called the move a 'further consolidation of the Kim family's power'.
State media is reporting that Mr Kim addressed the 'complicated international situation' during the party meeting.
He called the country's nuclear weapons a 'powerful deterrent firmly safeguarding the peace and security in the Korean peninsula and Northeast Asia'.
Pyongyang is attempting to develop a nuclear weapon capable of hitting the US mainland.
In the past few weeks they have launched two missiles over Japan and conducted their sixth nuclear test, escalating tensions on the Korean peninsula and with the United States.
Kim's comments were made before US President Donald Trump tweeted that diplomacy with Pyongyang had failed and that 'only one thing will work' when dealing with North Korea.
The US President's recent statements about North Korea have been criticised by the Labour shadow foreign secretary Emily Thornberry.
'He is playing games, he is using intemperate, completely inappropriate language in very dangerous times because the difficulty will be how will North Korea interpret that?' she told Sky News' Sunday with Niall Paterson.
'And there could be a miscalculation, a misunderstanding that could lead us into terrible circumstances.
Two of the men responsible for the North Korean leader's rocket programme, Kim Jong Sik and Ri Pyong Chol, were also promoted at the meeting.
North Korea's foreign minister, Ri Yong Ho, who called Donald Trump 'President Evil' in a speech to the UN, was promoted as well.