The president's trip comes amid heightened tensions with Iran -- just days after he called off a military strike there -- as he seeks to make a deal with Chinese President Xi Jinping to curb a raging trade war, and with no apparent progress toward Trump's goal of securing the denuclearization of North Korea.
Here are a few storylines to watch:
China trade talks
Whether the meeting will produce real progress or only ratchet up tensions remains to be seen. Trump has been careful not to set expectations ahead of the meeting, describing it as a wait-and-see situation even as he has expressed hope for progress.
"We'll see what happens. I think the meeting might very well go well. And, frankly, our people are starting the deal as of tomorrow," Trump told reporters on Wednesday. "The teams, they're starting to deal. So, we'll see."
He added, "China would like to make the deal, we'd like to make the deal, but it has to be a good deal for everybody."
U.S. negotiators said at the time that China had backtracked on some commitments for a prospective deal. The president then moved to more than double the rate on $200 billion worth in Chinese goods already subjected to tariffs.
Trump also has since threatened to slap a fresh round of tariffs on an additional $300 billion worth of Chinese exports. The president has not put a deadline on those additional prospective tariffs and has said that he wants to meet with Xi first before making any decision on whether to move forward with that threat.
Matt Goodman, a senior adviser for Asian economics at the Center for Strategic and International Studies, said there's the potential for the meeting between the two leaders to produce an extreme result: Either a grand bargain or total breakdown. But the most likely result, Goodman predicts, is that the two will walk away with a truce of sorts.
"The most likely outcome [is] that they're going to reach some sort of accommodation, a truce like that, and push this forward. It's not going to solve the immediate problems," Goodman said. "Even if we get a deal, it's unlikely to solve some of the deep structural differences between us in the role of the state in the economy, the governance of technology and data."
Whatever the outcome, a senior administration official said the president would walk away content.
"He's quite comfortable with any outcome of those talks," the official said. "So it's really just, you know, an opportunity for the president to maintain his engagement, as he has, very closely with his Chinese counterpart. Even as trade frictions persist, he's got the opportunity to see where the Chinese side is since the talks last left off."
In addition to the standoff over trade, there is fresh tension between the two countries over the Trump administration's decision to blacklist Chinese tech giant Huawei. Citing national security concerns, Trump has barred U.S. companies from selling products to Huawei.
Meeting with Putin
In what will be another closely watched encounter, Trump is scheduled to have a bilateral meeting with Russian President Vladmir Putin.
It will be their first encounter since the release of special counsel Robert Mueller's report, which further concluded Russia was engaged in a concerted effort to interfere in the U.S. election. In November, the two leaders were supposed to meet on the sidelines of the G-20 in Argentina before the U.S. canceled, citing Russia's seizure of 24 Ukrainian sailors and three ships. This meeting is moving forward, even as those sailors are now being put on trial in Moscow.
Trump and Putin have met a handful of times before, and each previous meeting sparked controversy in one way or another. On one occasion, Trump emerged seeming to accept Putin's denial of Russia's meddling in the 2016 election, contradicting his own intelligence community. Another time, Trump seized his interpreter's notes after the meeting. House Democrats are pressing the White House for answers about what has since happened to those notes, which they've argued should be preserved as presidential records.
When the two leaders sit down again this week, Europe Director of the Center for Strategic and International Studies Heather Conley said the meeting will once again be intensely scrutinized.
"Whenever President Trump and President Putin meet, there is a very strong domestic backlash after that meeting," Conley said. "But, in part, it's because there's a total lack of transparency about the topics of discussion and what the agenda is. And I think the president would have a better policy approach domestically if, again, there was clarity of what the agenda would be."
The president's latest meeting with Putin comes after he recently expressed openness to listening to foreign actors should he be offered dirt on political opponents in 2020 , which he said in an interview with ABC News Chief Anchor George Stephanopoulos.
"The questions for the president after that meeting will be on the foreign influence question, which he has repeatedly raised, that he would be willing to accept that information, which goes to the heart of the 2016 presidential election interference," said Conley.
Asked as he departed for Japan on Wednesday whether he would warn Putin against meddling in future U.S. elections, the president wouldn't say whether or not he would.
"I'll have a very good conversation with him. What I say to him is none of your business," Trump said.
Visit to the DMZ
While the stated purpose for Trump's trip to South Korea is to meet with South Korean President Moon Jae-in, the president also may travel to the demilitarized zone between North and South Korea.
The White House has not confirmed whether he will.
"We're not going into details of the president's schedule yet. There are some things that are still coming together," a senior administration official said.
While the White House won't say if the president will travel to the DMZ, a senior administration official said there are "no plans" for the president to meet with North Korean leader Kim Jong Un during the trip.
Asked directly on Wednesday whether he'll meet with Kim, Trump said, "I'll be meeting with a lot of other people, not quite him, but I may be speaking to him in a different form."
The two leaders continue to exchange letters, and the president sent Kim a new letter as recently as last week.
Trump had planned to visit the DMZ on his last trip to South Korea, but the idea had to be scrapped due to bad weather that made a helicopter trip there too risky.
Either way, the visit comes amid a flurry of diplomatic activity that has U.S. officials optimistic. China's president made his first state visit to Pyongyang, Kim dispatched his sister Kim Yo Jong to the DMZ to meet briefly with South Korean officials and Kim has exchanged positive, but nonspecific letters with Trump.
The South Korean president said in an interview with Korean news agencies on Wednesday that North Korea and the United States are engaged in "behind-the-scenes talks" to set up a third summit between Trump and Kim. The White House declined to comment on Moon's comments.
Tensions with Iran
While Iran may not be on the formal agenda at the G-20, the gathering of major allies in Japan will provide an opportunity for key world leaders to discuss the escalating regional tensions and plans to respond.
The meeting of allies comes just a week after the president pulled back from a military strike on Iran in retaliation for shooting down a U.S. drone that Iranians contend was in Iranian airspace, but that the U.S. says was in international airspace.
But even as the possibility of military conflict between the U.S. and Iran has been averted, at least in the short term, Conley said the challenge for the U.S. going into the G-20 is a lack of an agreement with allies on how to approach Iran, especially after the U.S. withdrew from the Iran nuclear deal last year.
"There is no allied coordinated unified effort vis-à-vis Iran and this is going to -- we don't even have the British in support of our position, which was clear during the U.K. state visit by President Trump," Conley said.
Sitting down with the Saudi crown prince
Trump also plans to meet with Saudi Crown Prince Mohammed Bin Salman for a bilateral meeting that the White House said would be an opportunity to discuss the escalating tension with Iran.
But beyond the security cooperation between the two nations in countering Iran in the region, the meeting stands out as the first formal meeting between the president and the crown prince since the killing of journalist and U.S. resident Jamal Khashoggi.
The encounter comes after the United Nations extrajudicial executions investigator released a report last week stating there is "credible evidence" to warrant further investigation into the role that Saudi leadership, including the crown prince, in Khashoggi's death.
Trump spoke by phone with the crown prince late last week but has said he did not raise the U.N. report in that conversation and further brushed off the report's recommendation that the FBI further investigate Khashoggi's death.
"I think it's been heavily investigated," Trump said, emphasizing the importance of the security and business relationship with Saudi Arabia.
"I'm not like a fool that says, 'We don't want to do business with them,'" Trump said in an interview with NBC News' Chuck Todd. "And, by the way, if they don't do business with us, you know what they do? They'll do business with the Russians or with the Chinese. They will buy -- we make the best equipment in the world -- but they will buy great equipment from Russia and from China."