Liberal politician Moon Jae-in's victory would end nearly a decade of conservative rule and bring a more conciliatory approach toward North Korea.
Former human rights lawyer and liberal candidate, Moon Jae-in, has won South Korea's presidential election by a landslide, exit polls predicted when voting closed on Tuesday.
The election was called to choose a new president after Park Geun-hye was ousted and indicted for corruption, and took place against a backdrop of high tensions with the nuclear-armed North.
According to the joint survey by three television stations, Moon, who backs dialogue with the North, received 41.4 percent of the vote, while conservative Hong Joon-pyo was far behind with 23.3 percent, and centrist Ahn Cheol-soo third with 21.8 percent.
"I feel the people's strong will to change the government, we can make it a reality only when we vote," Moon said after casting his ballot with his wife in western Seoul.
National elections are public holidays in South Korea and a record turnout was expected, with 75.1 percent of voters casting their ballots by 7:00pm, an hour before the polls closed.
Moon's supporters say he would "restore democracy, which has been so undermined by the Park government."
But the 64-year-old has been accused by his critics of being soft on North Korea and favouring more independence in relations with the US, Seoul's security guarantor with 28,500 troops in the country.
Moon also says he would be willing to visit Pyongyang to meet Kim for the revival of some of the inter-Korean projects shuttered by his predecessors, including the Kaesong joint industrial zone.