Kim also pledged that North Korea would follow a policy of non-proliferation of nuclear technology.
And he said North Korea is willing to improve and normalize relations with countries that have been "hostile" in the past.
"As a responsible nuclear weapons state, our republic will not use a nuclear weapon unless its sovereignty is encroached upon by any aggressive hostile forces with nukes," Kim said during the congress, the first in 36 years.
As recently as March, Kim was threatening a "preemptive attack" against South Korea and the U.S.
It's important to note that North Korea has pledged in the past that it would "never use nuclear weapons first."
It has also sought negotiations with the U.S. and others over its nuclear program.
So the question is: Does this statement today represent a new direction for North Korea?
Or is it more of the regime here trying to sow confusion and gain a tactical advantage over the U.S. and other world powers in the showdown over its nuclear programs?
In the run-up to the party congress here, Western analysts speculated that Kim would use the congress to turn the page on the nuclear issue, declaring in essence that North Korea has now accomplished its goal of developing a credible nuclear deterrent -- and would now seek to emphasize economic development.
Today's declaration is likely aimed in the first instance at China -- with the hope that the sanctions strangling the economy here, which is primarily enforced by China, will now be loosened.
The real intentions of the North Korean regime are difficult to read during the best of times.
And Kim is still very much a mystery to Western leaders and analysts.
His rhetoric and his deeds on the nuclear front have been highly aggressive -- reckless, even by North Korean standards.
But his father, Kim Jong II, often issued blood-curdling threats just before reaching out and seeking new negotiations.
That might be happening here. As usual, when it comes to North Korea--it's very hard to be sure of anything.