They are likely to discuss an arms deal as Russia faces a Ukrainian counter-offensive, a US official said.
South Korea’s defence ministry confirmed that Mr Kim’s armoured train entered Russia early on Tuesday morning.
It is now moving towards Vladivostok, where Russia is hosting the Eastern Economic Forum.
The meeting between the two leaders could take place as early as Tuesday local time – although the statement from the Kremlin said it would happen in “coming days.”
Mr Kim is accompanied by senior government officials, including military personnel, KCNA reported.
High on the meeting’s agenda is the possibility of North Korea providing Moscow with weapons to support its war in Ukraine, a US official earlier told CBS, the BBC’s partner in the US.
His train is rumoured to include at least 20 bulletproof cars, making it heavier than average trains. Its weight also means the train is very slow – it can only travel at around 59 km/h (37mph).
The White House has said it has new information that arms negotiations between Russia and North Korea are “actively advancing”.
National Security Council spokesman John Kirby earlier said Russia’s Defence Minister, Sergei Shoigu, had tried to “convince Pyongyang to sell artillery ammunition” to Russia during a recent visit to North Korea.
Russia and North Korea each have things that the other country wants, said Ankit Panda from the Carnegie Endowment for International Peace.
Russia will likely ask North Korea for conventional arms, including artillery shells and rocket artillery munitions in exchange for food and raw materials, and continuing support at international forums like the United Nations, he said.
“This could open up the possibility of North Korea transferring more sophisticated weaponry to Russia to allow Moscow to maintain and backfill its own stocks of conventional weapons,” he said.
Mr Kim’s last trip abroad was also to Vladivostok in 2019 for a summit with Mr Putin after the collapse of North Korea’s nuclear disarmament talks with then-US President Donald Trump.
It was the first time Mr Kim had opened the country’s doors to foreign guests since the Covid pandemic.Follow TKG on Twitter & Facebook