Ruling party candidate Lee Jae-myung conceded defeat and congratulated Yoon in a statement read at the party headquarters.
"I did my best but failed to live up to your expectations," he said. "All responsibility lies with me. I extend my congratulations to candidate Yoon Suk-yeol."
Yoon of the conservative People Power Party (PPP) emerged from his home in southern Seoul and thanked his supporters before heading to the party's headquarters.
With 98 percent of the vote counted, Yoon had 48.59 percent of the vote against Lee of the liberal Democratic Party (DP)'s 47.79 percent.
More about: South-Korea
