The congress, which ended on May 9 after four days, was the first in 36 years.
Young leader Kim Jong Un, who assumed power in 2011 after his father's sudden death, took on the new title of party chairman on May 9, media reported.
The promotion - his previous party title was first secretary - had been predicted by analysts who had expected Kim would use the congress to consolidate his power.
The DPRK has come under tightening international pressure over its nuclear weapons program, including tougher UN sanctions adopted in March backed by lone major ally China, following its most recent nuclear test in January.
The congress's decision on strengthening the capability of its nuclear weapons formalizes the DPRK's position.
It had already declared itself "a responsible nuclear weapons state" and disavowed the use of nuclear weapons unless its sovereignty is first infringed by others with nuclear arms.
"We will consistently take hold on the strategic line of simultaneously pushing forward the economic construction and the building of nuclear force and boost self-defensive nuclear force both in quality and quantity as long as the imperialists persist in their nuclear threat and arbitrary practices," KCNA said, citing the congress.
