The two presidents will discuss denuclearization and peace on the peninsula, Moon's spokesman Park Soo-hyun said Wednesday. This will be the third time Moon and Xi have met, with their last encounter at an Asian leaders' summit in Vietnam in November.
Beijing was strongly opposed to that deployment, set in motion by Moon's predecessor, impeached President Park Geun-hye. While Moon also voiced skepticism over the missile system before assuming power, analysts said ongoing North Korean missile testing made it difficult for him to roll back deployment.
China reacted to the roll-out with an unofficial embargo of South Korea, which hit the country's tourism industry and prompted Chinese consumers to boycott Korean brands.
Rough road
Relations between Seoul and Beijing, though, began to improve in recent months. The two countries issued a statement in late October that recognized the "great importance" of the bilateral relationship.
"China needs a smooth relationship with South Korea in order to maintain its own geostrategic interests over the Korean Peninsula," he said.
"China has great concerns about the US alliance in the region and if China can build good relations with one of those US allies, that can work to undermine (the alliance), in some sense."
Euan Graham, director of the International Security Program at Sydney's Lowy Institute, said continued North Korean nuclear and missile testing had been pushing South Korea and the US closer together, undermining efforts by China to improve relations and increase its influence in the region ahead of the THAAD deployment.
Korean diplomacy
US Presidential Donald Trump has repeatedly stated that China is key to unlocking the North Korea issue. Beijing is Pyongyang's major ally and trading partner, but many experts have cast doubt on the actual amount of influence Chinese officials have over their North Korean counterparts.
Song Tao's visit was the first by a Chinese official to North Korea in 2017, but there's been no mention of any meeting between him and North Korean leader Kim Jong Un.
"The two sides also exchanged views on relations between the two parties and the two countries, and on the Korean Peninsula issue and other issues of common concern," Xinhua said.
On Tuesday, a senior United Nations official arrived in North Korea for the first visit of its kind in six years. Jeffrey Feltman, the UN's undersecretary-general for Political Affairs, will meet with officials and discuss "issues of mutual interest and concern," during a trip that's scheduled to end Friday, the UN said.
CNN's Yuli Yang and Ben Westcott contributed reporting.