Kim departed on Monday afternoon on a private train accompanied by his wife Ri Sol Ju and top North Korean officials. He will be in China until 10 January, according to the Korean Central News Agency reports on Tuesday.
The visit comes amid reports that negotiations are underway for a second summit between Kim and US President Donald Trump.
The two met last June, the first such meeting for a sitting US president.
Speculation had grown on Monday that Kim was possibly making his way to China after South Korea's Yonhap news reported that a North Korean train had been seen crossing the border.
Dozens of security vehicles and officials blocked roads around the train station in the border town of Dandong.
Hotel guests in Dandong had also not been allowed to enter rooms that faced the border, with news outlet Kyodo calling this an "apparent move to prevent the train from being seen".
Both countries media confirmed the visit on Tuesday morning, and Kim's distinctive green and the yellow train arrived at a station in Beijing on Tuesday morning.
The train, the same one used during Kim's first visit to China, resembles the one used by his father Kim Jong-il during his visits to China and Russia in 2011.
A motorcade with heavy security was later seen driving through central Beijing.
Kim's visit, during which he is being accompanied by several leading North Korean officials, is his fourth to China in less than a year.
Tuesday is also reportedly Kim's 35th birthday, though his date of birth has never been confirmed by Pyongyang.
China is an important diplomatic ally for North Korea, and one of its main sources of trade and aid.
"Kim is eager to remind the Trump administration that he does have diplomatic and economic options besides what Washington and Seoul can offer," Harry J Kazianis, Director of Defense Studies at the Centre for the National Interest told Reuters.
Kim, unusually, did not meet Xi for the first six years of his leadership of North Korea.
But last year, he visited China three times. None of the trips was announced in advance.