Han Myeong-sook said she will resign as party leader, Yonhap News Agency reported. Speaking at a news conference at party headquarters, she promised to work hard as a member of the DUP in December's presidential race.
"I will take responsibility for the defeat in the general elections and step down as leader of the party," she said.
The DUP entered the campaign with hopes of winning a parliamentary majority, but it emerged from Wednesday's vote with only 127 out of 300 seats. The ruling Saenuri Party now has an outright majority of 152 seats.
Observers said the DUP fumbled the selection of candidates and mishandled the revelation that one of its candidates had a past history of making sexist remarks.