South Koreans are voting in a presidential election, called early after a huge corruption scandal brought down their former leader.
Liberal Moon Jae-in is the strong favourite with centrist Ahn Cheol-soo his nearest challenger.
If he wins the South will likely change course on North Korea, with Mr Moon wanting to increase contact with the rogue regime.
But corruption and the economy are greater concerns for domestic voters.
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 (11:00 GMT), with the winner expected to be announced soon after. The new leader is likely to be sworn-in on Wednesday.
Heightened tensions on the Korean peninsula in recent weeks have made the perennial worries over the South’s volatile neighbour a key issue.
Mr Moon, of the Democratic Party of Korea, has advocated greater dialogue with North Korea while maintaining pressure and sanctions.
Both he and Mr Ahn have urged US President Donald Trump to cool his rhetoric towards the North after his administration suggested it could take military action over Pyongyang’s weapons programme.
Hong Joon-pyo of the conservative governing Liberty Korea Party however has attacked Mr Moon’s approach, saying on Thursday morning that the election was a “war of regime choices”.
North Korea state media said it favoured a return to an earlier era of communication and co-operation known as the Sunshine policy, seen as an endorsement of Mr Moon who was part of the previous South Korean government which promoted that policy.