"It's a one time shot and I think it's going to work out very well", he said.
He described the meeting as a rare opportunity for Jong-un to do something "great" for his people and the world and said he felt "confident" he the talks would end well.
The summit has been portrayed as a "get to know you" meeting.
Singapore's foreign minister, Vivian Balakrishnan, says it is "all systems go" for the summit.
"I think within the first minute, I'll know", Trump said, prompting a reporter to question how he would know. "And I think we should make that clear".
The standard thinking goes that he needs quick help to stabilize and then rebuild an economy that has suffered amid a decades-long pursuit of nuclear bombs, and that the North Koreans see a unique chance to win concessions, legitimacy and protection from a meeting with a highly unconventional US president who's willing to consider options past American leaders would not.
"I was really disappointed to see President Trump just dismissed the idea of talking about human rights", says Kristof.
"You know the way they say you know if you like somebody in the first five seconds?" he added. Well, I think that very quickly I'll know whether or not something good is going to happen.
The one thing it is impossible for North Koreans to understand, however, is how big the difference in prosperity is between their country and developed nations like the USA and South Korea.
United States 'closed the door' on tariff talks — European Union trade official
Other measures Mexico imposed include 20-25% tariffs on cheese and bourbon and duties of up to 25% on steel products. Trump's trade and tariffs battles and the uncertainty of how the war will play out is adding to the farmers anxiety.
"This is a leader who really is an unknown personality", Trump added of Kim.
The leader of the free world is off to make peace in the East but the possibility of a trade war looms in the West.
The apparent choice to use the Air China aircraft instead also underlines the strength of the traditional alliance between the socialist neighbours ahead of Mr Kim's historic meeting with Donald Trump.
Kim Jong Un's motivation for a treaty may be, in part, linked to eventually getting US troops - 28,500 of them - out of the southern part of the peninsula; many believe that the North sees this as the first step in paving the way for a single Korea ruled by Pyongyang.
"We're going to raise every issue, every issue will be raised", he said. "There's probably an even better chance that it will take a period of time, it'll be a process".
It may be the strangest feeling in decades to descend on the Korean Peninsula - a wave of optimism, and not always of the cautious variety, when it comes to North Korea and its nuclear bombs. But nothing will be fully settled until China, which sent hundreds of thousands of troops into the North to prevent a quick US victory in 1950, and South Korea also sign off.
Before Trump became president, Rodman appeared twice on his "Celebrity Apprentice" show and praised the billionaire real estate developer on Twitter during the 2016 presidential campaign. Trump noted that he would know "within a minute" whether Kim was serious about giving up weapons.
It has since been reconfirmed and with days ago the United States president has returned to his fire-and-fury rhetoric.
After cutting short his participation over the icy reception, Trump veered within hours from espousing a great relationship with the US allies to tweeting that his host, Canadian Prime Minister Justin Trudeau, had been "Very dishonest & weak" in post-summit remarks.