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.
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.
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.