The ballot is a straight fight between Park Geun-Hye, the conservative daughter of assassinated dictator Park Chung-Hee, and her liberal rival Moon Jae-In, the son of North Korean refugees.
Opinion polls indicate the result could go either way today.
The eventual occupant of the presidential Blue House will have to deal with a belligerent North Korea, a slowing economy and soaring welfare costs in one of the world's most rapidly ageing societies.
Park, 60, was looking to make history by becoming the first female president of a male-dominated nation and the first to be related to a former leader.
Her father remains one of modern Korea's most polarising figures -- admired for dragging the country out of poverty and reviled for his ruthless suppression of dissent during 18 years of military rule.
He was shot dead by his spy chief in 1979. Park's mother had been killed five years earlier by a pro-North Korea gunman aiming for her father.
Moon, who was chief of staff to the late left-wing president Roh Moo-Hyun, is a former human rights lawyer who was once jailed for protesting against the Park Chung-Hee regime.
Polling booths opened at 6am on a bright, chilly winter's morning, with the temperature hovering around -10 Celsius.
Initial turnout among the 40 million-plus registered voters was strong, with 35 per cent having cast their votes by midday -- a sharp increase on the 2007 election.
Although North Korea has not been a major campaign issue, its long-range rocket launch last week -- seen by critics as a disguised ballistic missile test -- was a reminder of the unpredictable threat from across the border.