The singer claimed that while the "recovery took time", she is "OK and safe now".
In the post, she added that many of her fans "wonder what happened to me, where did I disappear to and why".
She wrote: "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."
Duffy added: "You wonder why I did not use my voice to explain my pain?
"I did not want to show the world the sadness in my eyes."
She also asked her followers to support her "to make this a positive experience".
The revelation comes after she spoke to a journalist about the incident in the summer, Duffy claimed, adding: "He was kind and it felt so amazing to finally speak."
She said that "you can only imagine the amount of times I thought about writing this".
Duffy, 35, last released an album in 2010 with Endlessly.
She said that she would be posting an interview "in the following weeks".