Exit polls in South Korea suggest voters have overwhelmingly chosen the liberal candidate, Moon Jae-in, as their next president.
Early results suggest Moon is leading with 41.4%, while his nearest challenger, conservative Hong Joon-Pyo, is trailing with 23.3%.
Moon favours greater dialogue with North Korea, in a change to current South Korean policy.
The early election was called after a corruption scandal led to the impeachment of the former president, Park Geun-hye, who had been accused of allowing her close friend to extort money from companies.
She denies the allegations but could still face further prosecution.
Moon, a former member of South Korea’s Special Forces turned human rights lawyer belongs to the Democratic Party of Korea and had unsuccessfully ran against former President Park in 2012 elections.
He has positioned himself as the man who can move the country on from the scandals of Park’s era.
“I feel that not only my party and myself but also the people have been more desperate for a change of government,” he said while casting his vote earlier on Tuesday.
Moon has advocated greater dialogue with the North while maintaining pressure and sanctions, in contrast to Park who cut almost all ties.
He has been critical of the two previous conservative administrations for failing to stop North Korea’s weapons development.
But while tensions on the Korean peninsula ensured the election was closely watched, for South Koreans the priority has been corruption and the economy.
Moon has also talked of reforming South Korea’s huge family-run conglomerates, known as chaebols, which dominate the domestic economy.