Collins, 67, said there were two times in 1973 where he exposed himself to the “young woman.”
According to Collins, the underage girl had come to visit and stay with him and his first wife, and after his wife had gone to sleep, he and the girl were watching TV together.
“And I took her hand and moved it in such a way that she was touching me inappropriately,” he said. “I knew that something unthinkably wrong had just happened that I couldn't take back.”
Collins also confessed to exposing himself to a teenage girl in 1982 and again in 1994.
The actor told Couric that after extensive therapy and religious consultation, he has not had an impulse to act out in such a way since 1994.
“I’m a flawed person,” Collins said. “And in the church, it’s actually one of the things I love about the church ... it’s really one of the main things about the Christian faith, is you know, Christ said in so many ways, ‘bring me that ... which about you is broken, bring it,’ and most people are broken in some way.”
Allegations of Collins’ wrongdoing first surfaced in October when a 2012 audio recording of his confession during a marriage counseling session with his estranged wife, Faye Grant, was released by the gossip website TMZ.
Collins has lost acting jobs as a result of the revelations and “7th Heaven” has been pulled from syndication. He also resigned from national board of SAG-AFTRA, the union that represents people who work in television and radio.
In a previous statement to People magazine, Collins had said the marriage counseling sessions were recorded without his or his therapist’s knowledge or consent.
"This came out. I didn't choose this to come out," Collins told Couric. "I didn't want to have to deal with these things publicly.”
“I had dealt with them very, very strongly, and committedly, in my private life, but I think, I’m a human being with flaws and I've done everything to address it,” he said.
In a statement issued to ABC News in response to the new interview, Collins' estranged wife, Grant, wrote, "I sincerely hope Steven gets the help he needs."
The couple's divorce proceedings have been moved to next year.
Watch more of Katie Couric's interview with Stephen Collins on “20/20,” Friday at 10 p.m. ET on ABC.
ABC News' Michael Rothman and Lauren Effron contributed to this report.