Witherspoon and actress Mindy Kaling both appeared on Winfrey's Super Soul Sunday for an interview which will appear in the next issue of O magazine.
The three women are costars in the new fantasy film A Wrinkle in Time, but their discussion took a very real turn when Winfrey asked her guests, "What's the most difficult decision you've had to make to fulfill your destiny?"
Witherspoon revealed she had been in an abusive relationship - without naming her abuser - and said that choosing to leave that relationship changed her life forever.
The Legally Blonde star explained that she had been the victim of verbal and psychological abuse, and choosing to leave the relationship made her a stronger person.
"I could never be the person I am today. ... It changed who I was on a cellular level, the fact that I stood up for myself," she told the host, adding that since leaving the relationship she's changed completely.
"People say to me that knew me then, 'You're a completely different person.' I didn't have self-esteem. I'm a different person now and it's part of the reason I can stand up and say, 'Yes, I'm ambitious' — because someone tried to take that from me before."
Witherspoon explained what finally pushed her to end the relationship, and after her dramatic confession the host shared her own experience with an abusive partner.