Arriving from Tokyo hours after North Korea test-fired a nuclear-capable cruise missile, Trump is due to meet Lee on Wednesday in Gyeongju, which is hosting the Asia-Pacific Economic Co-operation forum, before Thursday's talks with Xi.
Speaking to reporters aboard Air Force One, Trump dismissed the North Korea missile test and said he was focused squarely on his meeting with the leader of the world's second-largest economy.
"The relationship with China is very good. So I think we're going to have a very good outcome for our country and for the world, actually," Trump said.
He expected to reduce US tariffs on Chinese goods in exchange for Beijing's commitment to curb exports of fentanyl precursor chemicals, he said.
The US could halve the 20 per cent levies on Chinese goods it charges in retaliation for the export of such chemicals, the Wall Street Journal reported.
Addressing a summit of APEC CEOs in Gyeongju before his meeting with Lee, Trump said a trade deal with South Korea would be finalised "very soon", though officials on both sides have been downplaying the prospect of a breakthrough this week.
The allies announced a deal in late July under which South Korea would avoid the worst of the tariffs by agreeing to pump $US350 billion ($A531 billion) of new investments into the United States.
But talks over the structure of those investments have been deadlocked.
Trump has also pressed allies like South Korea to pay more for defence, and South Korea has sought reforms to US immigration laws to allow for more workers to build factories after a raid on a Hyundai Motor battery plant in Georgia.
The leaders will discuss trade, investment and peace on the Korean peninsula at talks on Wednesday, Lee's office said, a reference to engagement with North Korea.
Trump has made repeated calls for a meeting with North Korean leader Kim Jong-un, but there has been no public comment from Pyongyang.
Kim has previously said he could be open to talking if Washington stops pressing him to give up nuclear weapons.
Adding to the golfing gifts he received from Japan's leader in Tokyo on Tuesday, Lee will present Trump with a replica gold crown and award him with the "Grand Order of Mugunghwa", the country's highest decoration.
Trump was also treated to red-carpet welcome in the southern city of Busan earlier on Wednesday.
Skipping the main APEC summit, Trump plans a dinner with Lee and bilateral meetings with several countries' leaders, including China's Xi, before he departs on Thursday.
Negotiators from the world's top two economies hashed out a framework on Sunday for a deal to pause steeper American tariffs and Chinese rare earths export controls, US officials said.
The news sent stocks soaring to record peaks.
Trump said he would speak to Xi about Nvidia's state-of-the-art Blackwell AI chips, with sales to China a key sticking point in trade talks.
Trump's trip to South Korea concludes a whirlwind swing through the region, among the hardest hit by his tariff policies and increased US-China competition.
In Malaysia, he announced trade agreements and oversaw the signing of an expanded truce between Thailand and Cambodia after a border conflict.
In Tokyo on Tuesday, Trump lavished praise on Japan's first female Prime Minister Sanae Takaichi, welcoming her pledge to accelerate a military build-up and signing deals on trade and rare earths.
