Kim arrived in Dandong on Saturday through the New Yalu River Bridge connecting North Korea and China, the official said on condition of anonymity as they are not authorised to speak publicly about the matter.
Chinese Premier Li Qiang met Kim in the border city, another South Korean official familiar with the matter said.
Kim's visit comes a day before the anniversary of China joining the 1950-1953 Korean War.
Kim was expected to return to North Korea after Saturday's visit, South Korea's Yonhap news agency reported.
A South Korean official familiar with the matter told Reuters it was unlikely Kim would visit Beijing before high-level meetings of China's Communist Party from Monday.
China's foreign ministry did not immediately respond to a request for comment.
China and North Korea opened the 3km New Yalu River Bridge, their most significant land border crossing, earlier in October after a long delay, aiming to rejuvenate trade and commerce after years of strained ties and pandemic-related disruptions.
The bridge is expected to help move heavy equipment, raw materials and energy products that North Korea needs for development.
The opening is the latest sign of warming ties between Beijing and Pyongyang, after two summits between their leaders in the past 13 months.
Kim's last known visit to China was in September 2025, to attend the 80th Victory Day celebrations in Beijing, when he held a summit with Chinese President Xi Jinping.
Xi made a state visit to North Korea in June.
China, which fought alongside North Korea in the Korean War, is Pyongyang's main economic and diplomatic backer and accounts for most of its official trade.
However, North Korea has drawn closer to Russia since Moscow launched its full-scale invasion of Ukraine in 2022, which analysts say has reduced Pyongyang's reliance on Beijing.
US President Donald Trump has been eager to jump-start talks with North Korea, but US and South Korean officials see fading chances of him meeting Kim in 2026, Reuters reported on Friday.
China hosts a summit of the Asia-Pacific Economic Co-operation forum in November in Shenzhen, which Trump is expected to attend.
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