The rare political gathering, which Kim last hosted in 2016, has drawn international attention as he is expected to unveil a new five-year economic plan and address foreign policy, just two weeks before US President-elect Joe Biden takes office.
In his opening speech, Kim said the country had achieved a "miraculous victory" by bolstering its power and global prestige since the last meeting, referring to military advances that culminated in successful tests in 2017 of intercontinental ballistic missiles capable of striking the U.S. mainland.
But the five-year economic strategy he set forth in 2016 had failed to deliver, he said, calling for a boost in North Korea's self-reliance to tackle internal and outside challenges hindering its progress.
"The strategy was due last year but it tremendously fell short of goals on almost every sector," Kim said, according to KCNA.
On the global pandemic, Kim lauded party workers for ensuring "stable situations against the coronavirus from beginning to end."
They had "resolutely overcome difficulties in the face of an unprecedentedly prolonged, unparalleled global health crisis," he added.
North Korea has not officially confirmed any coronavirus cases, although it reported thousands of "suspected cases" to the World Health Organization.
South Korean authorities have said an outbreak in the North cannot be ruled out as it had active trade and people movement with China before closing its border last January.
The congress, the country's eighth and the second under Kim, was attended by 4,750 delegates and 2,000 spectators, Kim said.
In KCNA images, no one was seen wearing masks and participants did not sit apart, unlike at some other recent public events.
Kim was accompanied by top aides, including his sister and senior party official Kim Yo Jong, nominal head of state Choe Ryong Hae and Premier Kim Tok Hun.
The meeting will last some days, during which Kim is also expected to announce leadership changes - potentially involving his sister - and discuss other organizational, budget and audit issues.