Mr Kim gave an opening speech where he said the developmental goals set at the 2016 congress "were not met in nearly all areas to a great extent", according to the Korean Central News Agency.
The gathering is the first of its kind in five years, only the eighth in the nuclear-armed country's history, and comes weeks before US President-elect Joe Biden takes office.
During his speech, Kim said the surest and fastest ways to resolve the ongoing difficulties is further bolstering internal strength.
North Korea's state-run media say the ruling Workers' Party congress opened on Tuesday.
The Workers' Party congress, which was revived by Kim in 2016 after a 36-year hiatus, began on Tuesday as North Korea, one of the world's poorest countries, faces what Kim has called "huge challenges and difficulties" brought on by an economy hammered by pandemic-related border closings, a spate of natural disasters and harsh USA -led sanctions meant to stop the country from putting the finishing touches on its illicit nuclear-tipped missile program.
The gathering follows leader Kim Jong Un's recent acknowledgement that he had failed to create economic growth for the country.
While consolidating power with a series of military provocations and ruthless purges, Mr Kim has been trying to build a "man of the people" persona by openly admitting some of his failures - once a taboo in a country that reveres an errorless, godlike leader.
During a public speech marking the party's 75th anniversary in October, Kim fought back with tears as he thanked his people for enduring the triple blow to the economy.
Kim Jong Un revived the congress in 2016 as part of his efforts to increase the party's authority and cement his grip on power.
On the pandemic, Mr 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's official numbers state that it has no cases of the virus, but almost 12,000 people have undergone testing up to December 17 and tens of thousands have quarantined, according to the newspaper, citing the World Health Organization.
At the same time the North is more isolated than ever after closing its borders last January to protect itself against the coronavirus that first emerged in neighbour and key ally China.
Seoul's Unification Ministry in charge of cross-border affairs said it hopes the meeting will provide a chance to foster peace and advance inter-Korean ties.
The congress met in Pyongyang to determine "a fresh line of struggle and strategic and tactical policies", with thousands of delegates and observers in attendance, KCNA reported.
Mr Kim usually gives a New Year address which maps out blueprints for his diplomatic, military and economic policy but skipped it this year.
Analysts said he will likely give a message once the congress is over.
Mr 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. His son, Kim Jong Il, who died in 2011, never held a congress.
The North has not made public the exact schedule for the party meeting, but it could last multiple days given that the previous congress was held for four days.
