The successor to Yoon Suk-Yeol is to be elected, since the now deposed president was impeached and charged with treason due to his sudden declaration of martial law in December last year.
According to opinion polls, opposition leader Lee Jae-Myung of the Democratic Party has a clear lead over competitor Kim Moon-Soo of the People's Power Party, which holds the governing power.
The winning presidential candidate will be sworn in as president immediately on Wednesday.
The polling stations are expected to close at 8 pm local time, and analysts believe a winner can be announced before midnight (around 5 pm Swedish time).
More than 15 million people have already voted during a two-day early voting period last week, which corresponds to almost 35 percent of the 44.4 million eligible voters.
The now deposed president Yoon Suk-Yeol shocked the country when he declared martial law in December last year to protect the country from "communist forces".
He was deposed in early April when the Constitutional Court ruled in an impeachment trial that the declaration did not meet the legal requirements for a national crisis.
A criminal trial against him is ongoing. If he is convicted of treason, he can be sentenced to life imprisonment or the death penalty.
