Liberal Moon Jae-in is the strong favorite with centrist Ahn Cheol-soo his nearest challenger.
South Korea’s economic issues are a big concern for voters but the election could see a shift in policy towards North Korea.
Moon Jae-in wants to increase contact with North Korea in contrast to impeached President Park Geun-hye who cut almost all ties.
A record turnout is predicted, with numbers boosted by younger voters, as South Koreans choose from 13 candidates.
Polls close at 20:00 local time, with the winner expected to be announced soon after. The new leader is likely to be sworn-in on May 10.
Heightened tensions on the Korean peninsula in recent weeks have made the perennial worries over the South’s volatile neighbor a key issue.
Moon Jae-in, of the Democratic Party of Korea, has advocated greater dialogue with North Korea while maintaining pressure and sanctions.
Both Moon Jae-in and Ahn Cheol-soo have urged President Donald Trump to cool his rhetoric towards North Korea after his administration suggested it could take military action over Pyongyang’s nuclear program.
However, Hong Joon-pyo of the conservative governing Liberty Korea Party has attacked Moon Jae-in’s approach, saying last week that the election was a “war of regime choices”.
North Korea state media said it favored a return to an earlier era of communication and co-operation known as the Sunshine policy, seen as an endorsement of Moon Jae-in who was part of the previous South Korean government which promoted that policy.
All the candidates are promising to protect the fragile recovery in the country’s economy – the fourth largest in Asia – and to bring down youth unemployment, which remains stubbornly high.
There have been vows to reform the family-run conglomerates – chaebols – which dominate the domestic economy.
Whoever wins will have to tackle ties with China, which retaliated economically over the deployment of a US missile defense system in South Korea.
All candidates have been promising a break from the past as symbolized by the deeply unpopular Park Geun-hye.
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