Polling stations opened at 6 a.m. local time (11 p.m. CET) and will remain open until 8 p.m. (1 p.m. CET).
Voters are choosing among six candidates, though the race is largely seen as a two-way contest between liberal and conservative camps.
More than 44 million South Koreans are eligible to vote in the country of 52 million people. Over 14,000 polling stations have been set up nationwide.
Nearly 15.5 million citizens already cast their ballots in early voting held last Thursday and Friday, an option aimed at those unable to vote on election day.
The presidential election was triggered by the impeachment of President Yoon Suk Yeol, who was removed from office over alleged abuses of power.
In early December, Yoon briefly declared martial law, citing threats from North Korea - a move that was swiftly overturned by South Korea’s parliament.
The front-runner is liberal candidate Lee Jae-myung, who has consistently led in opinion polls with support hovering above 49 percent.
His main rival comes from the conservative bloc, making Tuesday’s vote a crucial moment for the country’s future political direction.
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Source: PR24/IAR/Reuters