The 48-year-old singer, whose real name is Melanie Chisholm, discussed the experience with Elizabeth Day on her podcast How To Fail while promoting her upcoming memoir Who I Am.
Also known by her persona Sporty Spice, Melanie C said the incident left her feeling “violated, vulnerable and embarrassed”.
She said: “We were in Istanbul, we did two shows and we had never done a full-length concert before, so obviously we’d rehearsed for weeks ahead.
“Everything was leading towards the pinnacle of everything I had ever wanted to do and ever wanted to be.
“What drives me is being on stage, being a performer. So, here we were the eve of the first-ever Spice Girls show, so I treat myself to a massage in the hotel and what happened to me I kind of buried immediately because there was other things to focus on.
“I didn’t want to make a fuss, but also I didn’t have time to deal with it. Because I didn’t deal with it at the time, I realised that I allowed that to be buried for years and years and years.”
The singer said the experience came to her in a dream while she was writing her memoir and she decided to include it.
“I think it’s really important for me to say it and to really deal with it and process it and for other people. Terrible things happen all the time and this situation wasn’t as bad as it could have been.
“In a version of sexual assault, it is a mild version but I felt violated, I felt very vulnerable, I felt embarrassed, 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 was so many thoughts and feelings, and I just thought I do want to talk about it because it has affected me.”
The book by the former Spice Girl singer will follow the journey from her bedroom in north-west England to global recognition with the group.
She is the last member of the group to publish her autobiography.
After coming together through a trade paper advert in 1994, the Spice Girls – Melanie C, Mel B, Victoria Beckham, Emma Bunton and Geri Halliwell – enjoyed a meteoric rise to fame, becoming internationally known for their message of Girl Power.
Melanie C’s book will detail the incredible highs of becoming one of the world’s most recognisable pop stars, including playing at Wembley, conquering the Brit awards and closing the Olympics.
She will also share the lows of struggling with her body image and mental health, and the difficulties of finding her own identity in a world where everyone knows your name.