The two leaders also shook hands over the border at the DMZ, AFP news agency reported.
In a historic moment, Trump walked across the concrete blocks dividing North and South Korean territory in the Demilitarized Zone that divides the peninsula, where the two sides fought each other to a standstill in the 1950-53 Korean War, AFP said,
North Korea's Kim Jong Un said that President Donald Trump's brief visit to North Korean territory improves ties between the new country, Associated Press reported.
The impromptu meeting in the DMZ - which came after Trump issued an invitation on Twitter on Saturday - comes with negotiations between Pyongyang and Washington over the North's nuclear arsenal at a deadlock, AFP said.
"If Chairman Kim of North Korea sees this, I would meet him at the Border/DMZ just to shake his hand and say Hello(?)!," he wrote from from Osaka in Japan, where he was attending a G20 summit before flying to Seoul.
he Trump-Kim meeting was confirmed earlier in the day by South Korean President Moon Jae-in at the Presidential Palace here during a joint press conference with his American counterpart, Yonhap reported.
Moon was speaking at a joint press conference with Trump at Cheong Wa Dae (Presidential Palace) in Seoul, after which the US President headed straight to the DMZ that separates the two Koreas.
Moon said he has been invited to the DMZ as well, but his "dialogue" with Kim will take place at a later time.
The focus of Sunday's DMZ event is dialogue between Pyongyang and Washington, he added.
Trump had said his upcoming meeting with Kim is "just a step". He said it "might be an important step, it might not. Probably it is a step in the right direction".
He earlier said he was seeking a "handshake" session, not a summit, at the DMZ.
Speaking to the media after the meeting, Trump, who is on a two-day visit to South Korea, said it was a "great honour to cross that line (DMZ)", while Kim deemed it "as a "historic moment".
Trump has also invited Kim to Washington, but it was not immediately clear if the latter has accepted the invitation.
If Kim accepts the invitation, it would be the first time a North Korean leader will visit the US. Following the meeting at the DMZ, the two leaders walked into South Korea.
During his previous visit to South Korea in November 2017, Trump pushed for a DMZ trip with Moon by chopper. But it was cancelled due to bad weather.
(with inputs from agencies)