The 35-year-old Welsh star posted on her verified Instagram account that her "recovery took time".
The performer, who had a UK number one single Mercy in 2008, wrote to her 33,000 followers: "The truth is, and please trust me I am OK and safe now."
"I was raped and drugged and held captive over some days," she wrote.
Duffy, whose debut album Rockferry went seven times platinum as it went to number one in six countries, won three Brit Awards and a Grammy following her breakthrough.
"You can only imagine the amount of times I thought about writing this," she wrote on Instagram.
"Well, not entirely sure why now is the right time, and what it is that feels exciting and liberating for me to talk.
"I cannot explain it. Many of you wonder what happened to me, where did I disappear to and why. A journalist contacted me, he found a way to reach me and I told him everything this past summer. He was kind and it felt so amazing to finally speak.
"The truth is, and please trust me I am OK and safe now, I was raped and drugged and held captive over some days. Of course I survived. The recovery took time. There's no light way to say it. But I can tell you in the last decade, the thousands and thousands of days I committed to wanting to feel the sunshine in my heart again, the sun does now shine."