The U.S. Vice President Mike Pence and Japanese Prime Minister Shinzo Abe represented their homelands.
Kim Yo Jong offered an invitation from her brother for Moon to visit the North, in the strongest sign yet of an expanding diplomatic opening opposed by the U.S.
Japan and South Korea share a bitter history that includes Japan's 1910-45 colonization of the peninsula and the use of comfort women, Japan's euphemism for women - many of them Korean - forced to work in its wartime brothels.
A North-South Olympics detente has raised concerns in Washington and Tokyo that Seoul may ease up on the "maximum pressure" campaign by the United States and its allies to rein in North Korea.
A massive military parade in Pyongyang on the eve of the just-opened Pyeongchang Games has been used as Exhibit A by skeptics.
Even so, there's also cautious optimism, or curiosity at least.
North Korea has spent years developing its military, saying it needs to protect itself from US aggression.
She is to arrive today on a private jet to attend the opening ceremony of the Pyeongchang games, and will join Moon for lunch Saturday, presidential spokesman Kim Eui-kyeom said. Pence and the Kims seemed to go out of their way not to acknowledge each other.
Kim's sister, Kim Yo Jong, verbally delivered the invite to Moon on Saturday.
The North Korean delegation is set to return home Sunday.
The opening part of the talks was mostly about the weather: Pyeongchang was colder than Seoul, it was agreed.
Moon did not appear until the fifth paragraph: "Attending it were President Moon Jae In and personages of political and social circles of south Korea".
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At the luncheon, Moon proposed a toast, calling for peace and "mutual prosperity" for the two Koreas. Then the air was filled with a well-known ballad of sorrow and separation.
The recent detente, anchored by South Korea's hosting of the Winter Olympic Games that began on Friday, came despite an acceleration in the North's weapons programs past year and pressure from Seoul's allies in Washington. Abe said that "a promise" made between the nations should be respected as a "cornerstone of the bilateral relationship". "But the Olympic moment that President Moon is trying to generate here is not a time to nurse those grievances", Jannuzi said.
This item corrects that Kim Yo Jong is the leader's sister.
The North Korean officials were first received by Moon's chief of staff, Im Jong-seok, who was standing outside of the main facility of Cheong Wa Dae. The Koreans were crushed by Switzerland, 8-0.
Moon's office says Kim's sister, Kim Yo Jong, came to the South as his special envoy.
USA officials have been urging South Korea to be cautious in its rapprochement with the North - a point Pence drilled home in private meetings with Moon on Thursday.
The pair of national leaders have been known to trade threats of nuclear war over the past year. He criticized the Trump administration for straining too hard to signal disgust of Kim Jong Un's government. Many in Seoul, however, while interested in the warming high-level contacts, are also tempering expectations for a real breakthrough. A big one could be a demand to cease the U.S.
But Moon, a liberal who has always wanted to engage the North, will also have to convince a good portion of his own people who are deeply wary about North Korean intentions. North Korea has jumped at the opportunity.
Even if there are more meetings between the rivals after the Games, accomplishing something is another matter.
Vice President Mike Pence is cheering on US speed skaters at the Winter Olympics, before departing South Korea for Washington on Saturday. Pence carried a similar message.
"Kim Jong Un is clearly serious about reviving talks with the South to improve relations", said Koh Yu-hwan, a North Korea expert at Seoul's Dongguk University and a security adviser to Moon.