The right-of-center news websites RedState and the Daily Mail published in October claims Hill engaged in a "throuple" with a campaign staffer and her husband, as well as a separate affair with her senior legislative director.
Hil resigned on Sunday, saying in a statement posted to Twitter that she made the decision because she was scared of what could follow.
"I know that as long as I am in Congress, we'll live fearful of what might come next and how much it will hurt."
—Rep. Katie Hill (@RepKatieHill) October 27, 2019
Hill wrote that she was resigning to protect her friends, family, supporters, staff, and children from further repercussions.
She said: "This is what needs to happen so that the good people who supported me will no longer be subjected to the pain inflicted by my abusive husband and the brutality of hateful political operatives…"
She called her estranged husband, Kenny Heslep, "a monster who is driving a smear campaign built around cyber exploitation."
Heslep, who is involved in divorce proceedings with Hill, has not commented on the situation, or answered allegations that he is responsible for the leaks.
On October 18, RedState reported Hill was involved in a two-year consensual sexual relationship with Heslep and a campaign staffer.
Heslep filed for divorce in July 2019 after the relationship broke down, RedState wrote.
RedState published screenshots of texts between Heslep and the staffer, as well as a nude photo of Hill.
On October 24, DailyMail.com also published nude photos of Hill and the campaign aide.
Hill confirmed the relationship with the aide in her statement:
"During the final tumultuous years of my abusive marriage, I became involved in a relationship with someone on my campaign. I know that even a consensual relationship with a subordinate is inappropriate, but I still allowed it to happen despite my better judgment. For that I apologize. I wish nothing but the best for her and hope everyone respects her privacy in this difficult time."
Hill threatened to sue the Daily Mail, alleging that it published nonconsensual pornography, known as revenge porn, which is illegal in California.
Hill's attorneys wrote a legal letter to the Mail on October 24, obtained by Politico, demanding that their article be taken down.
It said: "You have also exposed your publication to grave legal consequences for California has some of the strongest criminal laws in the United States against the secretive generation and distribution of private, sexual images."
The House Committee on Ethics said on Wednesday it was investigating the allegations, Reuters reported.