The early election had been called after a corruption scandal led to the impeachment of former president Park Geun-hye.
The scandal has left the country deeply divided, but, as ... reports, President-Elect Moon has promised to unify South Koreans.
In the South Korean capital Seoul, large crowds cheered on their new president, Moon Jae-in, as he waded through a sea of people to take the stage for his victory speech.
"From tomorrow, I will be a president for everyone. I will be the president that also serves all the people who did not support me."
The 64-year-old Democratic Party leader's victory ends nearly a decade of rule by those strongly opposed to working with North Korea's leaders.
Mr Moon, a former human-rights lawyer who has taken a more conciliatory position on North Korea, won decisively.
A joint survey by three television stations showed him winning around 40 per cent of the votes, almost double that of his closest competitor.
Hong Jun-Pyo, deemed the conservative, finished with around 23 per cent, and independent candidate Ahn Cheol-Soo trailed on 22 per cent.
Mr Hong, the Liberty Korea Party candidate, accepted the results shortly after exit polls forecast Moon Jae-in's victory.
"If the exit polls are true, I will just be satisfied with the fact that I re-established the Liberty Korea Party. I accept this time's election results and am satisfied with the re-establishment of the party. Thank you all."
A supporter of Mr Moon, 54-year-old Moon Jang-gil, says he is thrilled by the result.
"I feel very, very good. I'm so glad that people made Moon Jae-in president. Now we have the right to have a great president like Moon Jae-in."
Another supporter, 26-year-old Kwak Gi-yeon, says the election of the new president has restored her faith.
"I feel thrilled, and I'm excited. Actually, as I'm in between jobs, I never thought about my dreams or hopes. I thought they were useless. However, I saw what Moon Jae-in has done in this tough situation, so, finally, I can dream of hope again."
Moon Jae-in will lead a country still deeply divided after former president Park Geun-hye was ousted.
Her impeachment in 2016 was a culmination of claims of abuse of power, extortion, bribery and leaking state secrets, all of which she denies.
But the President-Elect insists he will restore unity in South Korea.
"It is the great victory of great people who have gone through together in order to make a just nation, a unified nation, and a nation where principles and common sense work."
The election comes amid reports North Korea is preparing for another nuclear test.
But Mr Moon's victory could mean a shift in attitude towards the hermit kingdom, with him advocating dialogue to ease tensions.
Some critics, however, accuse him of being too soft on the North.
And others have speculated the new president's position on North Korea could strain relations with the United States.
Mr Moon had previously questioned the US deployment of an anti-missile system in South Korea.
But a US official says, while his win "introduces a level of volatility," it is not going to significantly change the alliance.
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