The campaign has focused largely on jobs and the economy, with North Korea less prominent despite high tensions on the Korean Peninsula.
South Korea's presidential hopefuls made a final push for votes on Monday, with the left-leaning candidate a clear favourite, as North Korea assailed the outgoing conservative government a day before the polls.
A former pro-democracy activist and human rights lawyer, Moon Jae-in of the Democratic Party, who favours engagement with Pyongyang, has been leading opinion polls for months.
The final Gallup Korea survey of the campaign ahead of Tuesday's vote gave him 38 percent, far ahead of centrist Ahn Cheol-soo on 20 percent.
Tuesday's vote was called to choose a successor to Park Geun-hye after her impeachment for corruption and abuse of power.
The campaign has focused largely on jobs and the economy, with North Korea less prominent despite high diplomatic tensions between Washington and Pyongyang over its nuclear and missile ambitions.
Moon promises economic invigoration
Moon has promised to reinvigorate the South's sluggish growth and create more jobs and hinted at a more flexible approach towards its nuclear-armed neighbour.
The frontrunner advocates dialogue and reconciliation with the North to defuse the situation and eventually lure it into negotiations that have been at a standstill for years. His approach has been criticised by his conservative opponents.
Pyongyang on Monday slammed the South's conservatives, who have been in power for a decade, as "senseless traitors seeking only confrontation and war" who were responsible for the "tragic" state of North-South relations.
An editorial in Rodong Sinmun, the mouthpiece of the ruling Workers' Party of Korea, said conservatives were scheming to retain power.
The North has carried out two nuclear tests and a series of missile launches since the start of last year, and Washington has said military action was an option to curtain Pyongyang's ambitions, sending tensions spiralling.
Park corruption scandal benefits Moon
Moon, who lost to Park in the last election in 2012, has benefited electorally from the anger over the scandal that brought her down, which saw millions of South Koreans taking to the streets in candlelit demonstrations to demand her removal.
Ahead of a packed day of last-minute rallies across the country, Moon asked voters to deliver him a hefty mandate.
"With landslide support with tens of thousands of votes, a miraculous change like a natural cataclysm is possible," he said at his campaign headquarters in Seoul.
"I will work as the people's president from the day I am selected by the people."
Centrist Ahn similarly vowed to "head straight to work at the presidential office of Cheong Wa Dae" if he wins and address urgent issues regarding the country's security, diplomacy and economy.
In December Moon said that if elected, he was willing to visit North Korea ahead of the United States, the South's security guarantor with 28,500 troops based in the country.
The comments were seized on by opponents who accuse him of being soft on nuclear-armed Pyongyang. He sought to backtrack, saying he meant defusing tensions was an issue of utmost urgency and he would meet US President Donald Trump before any other leaders.
Hong puts security first
Hong, from the ousted Park's Liberty Korea party – who placed level with Ahn in some surveys last week, the last ones available under South Korean law – said security would top the agenda if he won.
"Tomorrow is judgement day for the pro-North, leftist forces," Hong said. "Please help me win by a landslide so it's impossible for them to pick a fight."
A high turnout is expected, even more than the last vote's 75.8 percent, with over a quarter of South Koreans already having voted in early ballots last week.