After a big corruption scandal brought down the former president, South Koreans are casting their votes in a snap election.
The Liberal candidate Moon Jae-in is the big favourite and centrist Ahn Cheol-soo the nearest challenger.
Policy towards North Korea and Economic issues are a big concern for South Koreans. Mr Moon wants to improve relationships with the North unlike the former president Park Geun-hye who severed a lot of links.
South Koreans can choose from 13 candidates this time. A record turnout is predicted as 63.7% of voters casted their ballots four hours before the polls closed compared with 59.3% at the same point five years ago.
Voting closes at 20:00 local time (11:00 GMT), with the winner expected to be announced shortly after. The new leader is likely to be sworn-in on Wednesday.