The election is scheduled on 3 June and voting precincts will be open from 06:00 local time (22:00 GMT) to 20:00. South Koreans overseas were allowed to vote early from 20 to 25 May.
Results are expected to come in after polls close and the winner will likely be known in the early hours of the following day.
When Yoon defeated Lee in 2022, he was proclaimed the winner nine hours after the close of voting, or at 04:40 the morning after election day.
That was the closest presidential contest in the country’s history, which saw Yoon win by a 0.73% difference in votes.
The new president will take office immediately and unlike many of his predecessors, will not have the advantage of a formal transition from Yoon.
Yoon faces trial for an insurrection charge as a result of his attempt to impose martial law.
In January this year he became South Korea’s first sitting president to be arrested after investigators scaled barricades and cut through barbed wire to take him into custody. He was released from detention weeks later on a technicality.
He was also recently indicted for abuse of power, a separate charge to insurrection.
Before the election, Yoon quit his party in what analysts said was an attempt to shore up the chances of PPP’s presidential candidate Kim Moon-soo.