Left-leaning Moon Jae-In of the Democratic Party has held what appears to be an unassailable lead in opinion polls for months.
Victory will cap a political career that began with student activism in the days of military rule, when he was convicted of taking part in illegal protests.
The election comes after millions of South Koreans took to the streets in candlelit demonstrations to demand the removal of Park Geun-Hye, who was sacked by the country's top court in March over a corruption scandal and is now in custody awaiting trial.
"Our efforts to create a country worth living in started with candlelights and must end with votes," Moon told an audience this week.
The irony is that he was once chief of staff to liberal president Roh Moo-Hyun, who committed suicide in 2009 after being questioned over graft allegations.
"Corruption is the biggest issue in South Korean politics," says Robert Kelly of Pusan National University. "That's absolutely true. Every South Korean president has gotten into trouble for corruption and bribery and graft and things like that, of varying degrees."
But Moon boasts a clean image himself, said Kim Neung-Gou, president of online newspaper Polinews, and has been "riding on waves of protests against Park and accumulated corruption".
