In a Facebook post, Ahn "begged forgiveness" for his "foolish behaviour" and said denials issued by his office were untrue.
"It is all my fault. I seek forgiveness for my foolish act," he wrote, admitting his sexual misconduct.
The woman, Kim Ji-eun, appeared on local television network JTBC's news programme on Monday evening and made her allegations public. She said she planned to file a legal complaint against Ahn on Tuesday, the Korean Herald reported.
Kim accused the 54-year-old of raping her four times over the course of eight months, from June 2017 to February, and said he had sexually harassed her on many occasions.
She also alleged there were other victims in Ahn's office. Kim said the #MeToo campaign, sparked by the widespread allegations of sexual abuse in Hollywood, had given her the courage to come forward.
The campaign, which has seen thousands using the hashtag to share their experiences of sexual harassment, has spread across the globe.
It was slow to take off in South Korea, but recently gained ground after a number of women came forward.
Ahn's office had initially claimed that he had consensual sex with Kim. However, later Ahn said his office had "falsely" announced that the relationship was consensual.
Ahn worked as a political aide to the late President Roh Moo-hyun during Roh's presidential campaign in 2002. He ran against current President Moon Jae-in to be the Democratic Party of Korea's nominee for the 2017 presidential elections.
Though he lost, he was widely seen as a leading candidate to replace Moon in the 2022 elections.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
