The country's de facto head of state Kim Yong-Nam announced the post as foreign journalists briefly watched proceedings at the party congress -- the first for 36 years.
North Korea stages once-in-a-generation party congress
Kim -- the third generation of the dynasty that has ruled the country since its founding in 1948 -- was previously first secretary of the party.
The appointment will further bolster his authority at a congress seen as a coronation of sorts for the young leader, who came to power in December 2011 after the death of his father Kim Jong-Il.
Kim Jong-Il has been designated eternal party general secretary.
North Korea party congress pushes nuclear weapons expansion
The current leader's grandfather and the country's founding leader, Kim Il-Sung, remains eternal president of the nation despite having died in 1994.
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