Moon Jae-in was sworn in as the new South Korean president in the Parliament Building, hours after winning a landslide victory in the five-way presidential race, a media report said on Wednesday.
Moon's term as the country's new head of state began only one day after the nation held the unprecedented presidential by-election, Yonhap news agency said.
The new presidency began even before an inauguration ceremony as his election was a result of the rare poll that sought to fill the top elected office vacated by former President Park Geun-hye.
Moon's five-year term began at around 8 a.m. when the National Election Commission (NEC) confirmed his victory.
The liberal candidate from the Democratic Party secured 13,423,800 votes, or 41.08 per cent of the total 32,807,908 votes cast, according to the NEC.
A written certificate confirming Moon's election was presented to the new President.