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".
The test, which was followed a month later by a successful long-range rocket launch, "clearly demonstrated to the whole world our undefeatable spirit and endless power ... in defiance of malicious pressure and sanctions by enemy forces", he said.
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.
-- 'New milestone' --
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".
It is still unclear how many days the congress will last, but the speeches and delegate reports will be scrutinized for any sign of a substantive policy shift, especially on the economic front.
Analysts will also be watching for personnel changes as Kim looks to bring in a younger generation of leaders hand-picked for their loyalty.
Around 130 foreign journalists were invited to cover the event, but were not allowed inside the congress venue on Friday, restricted instead to watching from a spot 200 metres (yards) away in the light drizzle falling on the capital.
The 1980 event was staged to crown Kim's father Kim Jong-Il as heir apparent to his own father, the North's founding leader Kim Il-Sung.
The 2016 version was held inside the imposing April 25 Palace, whose stone facade was adorned with huge portraits of the two late leaders, along with giant red and gold ruling party banners.
While the agenda -- and even the duration -- of the congress remains unknown, its main objective is clearly to confirm Kim Jong-Un's status as absolute ruler and the legitimate inheritor of the Kim family's dynastic rule which spans almost seven decades.
It may also enshrine as formal party doctrine Kim's "byungjin" policy of pursuing nuclear weapons in tandem with economic development.
-- Chinese absence --
Notably absent was any Chinese representation -- a possible reflection of the increasingly strained ties between the North and its sole major ally.
A large Chinese delegation had attended the 1980 gathering, headed by Li Xiannian, later China's official head of state.
The North Korean capital was immaculately primped and primed for the congress , with national and Workers' Party flags lining its broad rains wept streets, along with banners carrying slogans such as "Great comrades Kim Il-Sung and Kim Jong-Il will always be with us".
Preparation had involved mobilizing the entire country in a 70-day campaign that New York-based Human Rights Watch denounced as a mass exercise in forced labour.
In his speech, Kim described the campaign as an extraordinary feat that had helped the country meet its economic target for the first half of the year.
Since Kim took power after the death of his father in December 2011, North Korea has carried out two nuclear tests and two successful space rocket launches that were widely seen as disguised ballistic missile tests.
Even as the international community responded with tougher sanctions, he pressed ahead with a single-minded drive for a credible nuclear deterrent with additional missile and technical tests.
He also demonstrated a ruthless streak, purging the party, government and powerful military of those seen as disloyal, and ordering the execution of his powerful uncle, and one-time political mentor, Jang Song-Thaek.