Kim and his entourage departed from Beijing Railway Station on his special armored train later a little after 2 p.m. Wednesday, following the luncheon.
He also credited Chinese President Xi Jinping with helping reduce regional tensions, saying 'the Korean Peninsula situation has been easing since previous year, and China's important role in this process is obvious to all'.
North Korean state media reported Thursday that Xi accepted Kim's invite to visit, but this was conspicuously absent from Chinese media reports.
After his surprise visit to China, North Korean leader Kim Jong-un left with backing for a possible second summit with US President Donald Trump, state media said.
South Korea's Ambassador to the US says he is hopeful that a second meeting between President Donald Trump and North Korean leader Kim Jong Un will take place.
But since North Korea's rapprochement with South Korea and the U.S., Kim and Xi have thawed their frosty relationship, as each wants to negotiate with Trump on a key topic: Kim on denuclearization and Xi on trade.
Xi re-asserted China's importance in the diplomatic process surrounding the Korean peninsula, Xinhua said, quoting him as saying that Beijing stands ready to "play a positive and constructive role in maintaining peace and stability and realizing denuclearization on the peninsula and lasting peace and stability in the region".
Kim spent around half an hour touring a pharmaceutical factory operated by Tong Ren Tang, the largest producer of traditional Chinese medicinal products. For Xi, whose country provides a lifeline to North Korea's anemic economy, the meeting with Kim reminds Trump that China's co-operation will make any nuclear deal more effective.
The unannounced trip has been largely shrouded in secrecy. But others say Moon simply made a symbolic, conciliatory gesture toward North Korea.
The visit coincides with what South Korean officials say is Kim's 35th birthday on Tuesday.
"Considering the difficulties faced by South Korean businessmen of the Kaesong Industrial Park and the hopes of South Korean people who wish to visit our people's great mountain, we are willing to resume the Kaesong Industrial Park and the tours to Mount Diamond with no preconditions or strings attached", Kim stated.
DPRK is the acronym for North Korea's official name, the Democratic People's Republic of Korea.
All sides should "jointly push for a comprehensive resolution of the Korean Peninsula issue", he said.
"The Sino-US trade negotiations are a matter between China and the US".
"The crux is how to let North Korea denuclearise and to give it security guarantees. This means that China implemented the United Nations resolution well", he said.
Pyongyang needed to take "bold, practical measures for denuclearisation" to ensure sanctions are lifted, he told reporters, but added that "corresponding measures" were also needed from the USA, such as agreeing a "peace regime" and formally declaring an end to the 1950-53 Korean War.
However, a meeting between Mr Pompeo and senior North Korean official Kim Yong Chol has yet to be rescheduled after an abrupt cancellation in November.
"But US forces in South Korea or strategic assets in places like Guam and Japan are not linked with North Korea alone, as they exist for the stability and peace of the entire Northeast Asia", he said.
Multiple diplomatic sources have said that the Vietnamese government has said it desires to host the second North-U.S. summit.