Unconventional and proud of it, President Trump used Twitter to set up his historic meeting Sunday with North Korean leader Kim Jong-un.

Before arriving in South Korea from Japan, where he was attending the Group of 20 summit, Mr. Trump tweeted a public invitation to the reclusive Mr. Kim.

“I will be leaving Japan for South Korea While there, if Chairman Kim of North Korea sees this, I would meet him at the Border/DMZ just to shake his hand and say Hello(?)!” Mr. Trump tweeted.

It was an unconventional offer, like a high-stakes diplomatic version of a friend texting another to meet up at the diner.

But it worked. The North Koreans responded to U.S. officials, sparking “will he-won’t he” drama and hand-wringing over the lack of preparation — assuming the showcase hadn’t been planned in secret.

“He saw it, social media — pretty powerful thing, social media,” Mr. Trump said.

About 24 hours after the president’s tweet, Mr. Trump and Mr. Kim were shaking hands across the boundary between North and South Korea, the world’s most-militarized, and Mr. Trump became the first sitting U.S. president to set foot in North Korea, if only a few steps.

As they spoke in front of reporters at the border, Mr. Kim told the president, “Some people think as if this meeting was prearranged through the letters you have sent me. But [I] was surprised yesterday morning when you expressed a willingness to meet with me here, and also when we got the official confirmation late yesterday afternoon.”

The president said he decided to take a chance, as long as he was in the neighborhood.

“I didn’t really expect it,” he said. “We were in Japan for the G20. We came over and I said, ‘Hey, I’m over here. I want to call up Chairman Kim.’ And we got to meet.”

Mr. Kim said he probably wouldn’t have responded to the tweet if he and Mr. Trump hadn’t already been acquaintances.

“If it was not for our excellent relation between the two of us, it would not have been possible to have this kind of opportunity,” Mr. Kim told the president. “So I would like to use this strong relation to create more good news, which nobody expects.”

Mr. Trump thanked his counterpart for accepting his challenge.

“When I put out the social media notification, if he didn’t show up, the press was going to make me look very bad,” the president said of Mr. Kim. “So you made us both look good, and I appreciate it.”

He called their meeting, following a failed summit in Vietnam in February, a “special moment.”

Mr. Trump, a showman who understands the power of televised images perhaps better than any president since Ronald Reagan, praised “the power” of Mr. Kim’s voice on camera.

“Nobody has heard that voice before,” the president told reporters. “He doesn’t do news conferences, in case you haven’t heard.”

The president also suggested that he didn’t know what would happen when he met Mr. Kim at the demarcation line between North and South.

“It was an honor that you asked me to step over that line,” he said to Mr. Kim. “And I was proud to step over the line. I thought you might do that; I wasn’t sure. But I was ready to do it.”

He added, “I think the relationship that we’ve developed has meant so much to so many people.”

• Tom Howell Jr. reported from Seoul.

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