Last polls place left-leaning Moon Jae-in as the front runner. Elections are taking place with a public eager for a fresh start after months of protests to first impeach and then indict Par Geun-hye for corruption.
Polling stations opened in South Korea's presidential election at 6 am local time Tuesday (2100 GMT Monday), to choose a new leader after Park Geun-Hye was ousted and indicted for corruption.
Turnout is expected to hit a record high with voters galvanised by anger over the sprawling bribery and abuse of power scandal that brought down Park.
South Koreans are voting for a new president amid widespread expectations of victory for a liberal candidate.
Left-leaning Moon Jae-in, a former human rights lawyer, has held a commanding lead in opinion polls for months, with the final Gallup Korea survey before a week-long pre-election blackout giving him 38 percent support.
Moon's nearest rivals are Ahn Cheol-soo, a centrist who has shown a more conservative stance on North Korea, and Hong Joon-pyo, a member of Park's embattled party who has called for the reintroduction of US tactical nuclear weapons in South Korea.
Moon has been the clear favourite as the country's powerful conservative forces struggle to regroup after a huge corruption scandal that led to Park's removal from office and arrest in March.
TRT World's Joseph Kim takes a look at the candidates in play.
What would a Moon victory bring?
Analysts say a victory by Moon might cause friction with the United States because he advocates engaging North Korea. Moon has called Park's hard-line North Korea policy a failure. If elected, he says he'll employ both pressure and dialogue to persuade the North to abandon its nuclear ambitions.
"This is the last challenge of my life. I've really done my best so far. I've made enormous preparations. I'm confident. I'll strain every nerve to the last minute to be a president for all the people," Moon, 64, said on the eve of the election.
His victory would end a near decade of conservative rule by Park and her predecessor, Lee Myung-bak.
When the liberals were last in charge in Seoul, Moon served as chief of staff for then-President Roh Moo-hyun. They sought closer ties with North Korea by setting up large-scale aid shipments to the North and by working on now-stalled joint economic projects.
He also advocates building up a more assertive South Korea and is critical of Park's decision to allow Washington to install the Terminal High-Altitude Area Defense (THAAD) advanced anti-missile system in the South. The system has irked Beijing, Seoul's largest trading partner.
Following a stand-off between Trump and North Korean dictator Kim Jong-un over Kim's reported nuclear test plans, Moon has talked more about bolstering national defence and said THAAD deployment is inevitable if North Korea provokes. Critics say Moon was looking to woo conservative voters.
"I'll take charge so you won't have to worry about security, national defence and peace," Moon said Monday in a message addressed to senior citizens, many of them conservative voters who oppose Moon because of what they see as a soft North Korea policy.
Even if he becomes leader, many analysts say Moon won't likely pursue drastic rapprochement policies because North Korea's nuclear program has achieved too much progress since he was in the Roh government a decade ago. Foreign experts say it may take only several years for North Korea to develop nuclear-armed missiles that can reach the US mainland.
After the polls close
Voting stations are to close at 8 pm local time.
South Korean TV stations plan to release the results of their joint exit polls soon after the vote ends and are expected to predict a winner before midnight.
The winning candidate will be officially sworn in as South Korea's new president after the National Election Commission ends the vote count and confirms the winner on Wednesday.
This forgoes the usual two-month transition because Tuesday's vote is a by-election to choose a successor to Park. Her term was originally meant to end in February 2018.
The new leader will still serve out a full, single five-year term.
The Park scandal
Park, South Korea's first female president, is currently jailed at a detention facility near Seoul and awaits a criminal trial set to start later this month. She has been indicted on bribery, extortion and other corruption allegations that could theoretically send her to jail for life.
The allegations incensed many in South Korea, with millions taking to the streets and calling for her ouster.
Park sympathisers later staged their own rallies.
Dozens of high-profile figures, including Park's long-time confidante, Choi Soon-sil, and Samsung's de facto leader, Lee Jae-yong, have been indicted along with Park.
The drama gave Moon, who lost the 2012 election to Park by a million votes, a boost in his push to re-establish liberal rule.
Frequently appearing at anti-Park rallies, Moon called for her ouster and reform measures to clean up social inequalities, excessive presidential power and corrupt ties between politicians and business leaders.
Many of these legacies dated back to when South Korea was ruled by Park's dictator father, Park Chung-hee, a deeply divisive figure whose 18-year rule was marked by both rapid economic rise and severe civil rights abuse.