The Spice Girls alum — known to fans as the iconic Sporty Spice — detailed the incident in her forthcoming memoir THE SPORTY ONE: My Life as a Spice Girl, out Sept. 27.
In an excerpt, Chisholm, 48, revealed that in 1997 she was sexually assaulted by a massage therapist at a spa in the Istanbul, Turkey hotel she and her fellow Spice Girls were staying in. They called the reception desk to explain what happened, however, he had already left.
She shared more on the latest installment of Elizabeth Day's How to Fail podcast.
Jeff Kravitz/FilmMagic Spice Girls
"We'd never done a full-length concert before, so obviously we'd rehearsed for weeks ahead, costume fittings, make-up, hair, everything was leading to the pinnacle of everything I ever wanted to do, and ever wanted to be," Chisholm recalled.
She added that she's motivated by "being on stage, being a performer" so she decided to "treat myself to a massage" on the eve of the group's first concert.
"And what happened to me — I kind of buried it, immediately, because there were other things to focus on," she said. "I didn't want to make a fuss but also I didn't have time to deal with it."
When she began writing her memoir 25 years later Chisholm said she decided it was "finally" time to address it.
"It came to me in a dream, or I kind of woke up and it was in my mind," she said. "And I was like, 'Oh my gosh, I haven't even thought of having that in the book.' Then, of course, I had to think, 'Well, do I want to reveal this?' And I just thought, actually, it's really important for me to say it, and to finally deal with it and process it."
Though she described her experience as a "mild version" of sexual assault, she shared, "but I felt violated. I felt very vulnerable. I felt embarrassed."
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Chisholm said she also questioned herself. "And then I felt unsure — have I got this right? What's going on? I was in an environment where you take your clothes off with this professional person."
"So there were so many thoughts and feelings, and I just felt, you know what, I do want to talk about it because it has affected me. But I'd buried it, and I'm sure lots of men and women do," said the star.
As she prepares to release her book, Chisholm has also opened up about how the Spice Girls' success impacted her health.
"I was quite unwell for a few years," she said of her time in one of the biggest-selling groups of the '90s.
"When I look back, I don't know physically how I did it; when you consider how little I lived on and how much exercise I was doing alongside a brutal schedule."
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Chisholm was diagnosed with clinical depression, anorexia, a binge-eating disorder, severe anxiety and agoraphobia, according to The Daily Mail. She was put on antidepressants.
These days, Chisholm — who noted last year that another Spice Girls reunion isn't totally out of the question — said she sees herself as a "warrior."
If you or someone you know has been sexually assaulted, please contact the National Sexual Assault Hotline at 1-800-656-HOPE (4673) or go to rainn.org.