Hailing the historic test of what North Korea claims was a hydrogen bomb in January, the isolated state's young leader said it had shown the world it would not be cowed by sanctions or outside pressure.
Dressed in a western style suit and tie, Kim's speech was delivered to thousands of party delegates who had gathered in Pyongyang for the once-in-a-generation conclave.
In particular, he praised the country's scientists for "creating milestone miracles with the magnificent and exhilarating sound of the first H-bomb of our republic".
Most experts have questioned the North's H-bomb claim, saying the detected yield from the January test was far too low for a full-fledged thermonuclear device.
There has been widespread speculation that the North might have prepared another nuclear test to coincide with the congress, as a defiant gesture to underscore its nuclear power status.
The 33-year-old Kim, who was not even born when the last Workers' Party Congress was held in 1980, said the party conclave would prove to be a "new milestone" that would lay out the future direction "of our revolutionary march".
Analysts will be watching for personnel changes as Kim looks to bring in a younger generation of leaders hand-picked for their loyalty.
The state TV announcer said the congress had also discussed the operations of its powerful central military committee, revisions of party rules and elections to central party organs.
Around 130 foreign journalists had been invited to cover the event, but were not allowed inside the congress venue.
The 2016 version was held inside the imposing April 25 Palace in Pyongyang - its stone facade adorned with huge portraits of the two late leaders, along with giant red and gold party banners.
