Asked if he would meet Kim in Singapore, he said he thought the North Korean leader had "bigger things to worry about", but added that every time he'd met with Kim, it had been a surprise.
Last week, Trump told reporters that Rodman had not been invited to the summit, but praised him as a "nice guy" and great rebounder.
In his latest tweet, hours before the summit was due to begin, Mr Trump said meetings between the two sides' officials were "going well and quickly".
"This is more of a case of 'we'll believe it when we see it, ' rather than actually reacting", said Brad McMillan, chief investment officer for Commonwealth Financial in Waltham, Massachusetts.
Many experts on North Korea remain sceptical Kim will ever completely abandon nuclear weapons, believing his engagement aims to get the United States to ease crippling sanctions. With the eyes of the world upon them, here is a breakdown of how the day will unfold.
But after a flurry of diplomatic overtures eased tension in recent months, the two leaders are now headed for a history-making handshake that USA officials hope could eventually lead to the dismantling of a North Korean nuclear programme that threatens the United States.
Trump spoke only briefly in public on Monday, forecasting a "nice" outcome.
Kim's main concern is the survival of his own regime - South Korean President Moon Jae-in told reporters that the Pyongyang leader had "concerns on whether he could trust that the USA would end its hostile policy and guarantee the security of his regime when the North denuclearises itself".
North Korea has called on China to set up an emergency alert along the two countries' border NK-China border during the much-anticipated summit between U.S. President Donald Trump and the North's leader Kim Jong Un, sources inside the isolated nation said. If he resumes talking about his "maximum pressure" campaign to squeeze Kim's regime, he said last week, "you'll know the negotiation did not do well".
Tuesday's meeting, convened at a luxury hotel on the island of Sentosa, came just three months after Trump accepted North Korea's invitation for talks on the spot.More news: Trump congratulates Sanford challenger for win
The two men are expected to share a handshake before they meet alone with a pair of interpreters for roughly 45 minutes while their entourages wait nearby.
After the intimate huddle, they're scheduled to hold a larger meeting and working lunch.
For the first time since the division of the Korean Peninsula, the leaders of the two countries held a summit in June 2000.
But, as it stands now, North Korea is still a dictatorship, having been led by one family since September 9, 1948.
He accompanied the young leader at all four of his recent summits, twice with South Korean President Moon Jae-in and twice with Chinese President Xi Jinping.
The president is now in Singapore, just hours away from his historic meeting with Kim.
The Trump-Hannity chat will air at 9 p.m. Tuesday on "Hannity". Mr Kim is reported to be flying out even earlier, at 14:00 local time (6pm NZT). The selfie with Balakrishnan might draw raised eyebrows in the west, but it wasn't the image leading North Korean state media coverage of Kim Jong-un's trip to Singapore. Former South Korean President Kim Dae-jung met with former North Korean leader Kim Jong Il for two days in Pyongyang and the leaders produced an eight-point peace agreement.
There's excitement surrounding the summit but also skepticism that the North will relinquish a nuclear weapons program it spent decades building despite crushing sanctions.
Others felt the sit-down between Trump and Kim did little to change the game.