Adichie said the unnamed individual assaulted her in his office in Lagos when she went to seek his help for a book launch.
She made the disclosure on Tuesday while delivering the closing speech at the Stockholm Forum on Gender Equality in Sweden.
Narrating the incident, Adichie said: “When I was 17-years-old, I wrote a book of really bad poetry which I hope no one will ever read. But true to the delusional ambition of youth, I thought that it was a wonderful book.
“And in Nigeria, when a book is published, it is customary to have a book launch, to introduce the book to the public. And so I set about planning a book launch for this terrible book.
“There was a powerful man in the media who I knew could help with this book launch. And so I found my way to his office in Lagos and I told him about my book. Would he please support the book? I asked.
“He was very impressed that while other teenagers were hardly reading at all, I was serious and focused enough to have written a book.
“He was pleasant, warm and then he got up from his desk and walked around to where I was seated, and he stood behind me, and in a move that was as swift as it was shocking, he slipped his hands under my buttoned-down shirt, under my bra and squeezed my breast.
“I was so taken aback that I did nothing for seconds. Then I pushed his hands away, but gently, nicely because I did not want to offend him.
“Later that day, I broke into a rash on my chest, my neck, my face, as though my body were recoiling, as though my body was saying what my lips were not saying.
“I felt a deep loathing for that man and for what he did. I felt as if I didn’t matter, yet I told no one about it and I kept talking to him, being polite, hoping he would help.
“I was a feminist long before I knew what the word meant. I did not read feminist text. I knew that the world will not give to women the same dignities it gave to men.
Adichie went on to talk about how women are expected to be lesser than they are in a bid to make men feel secure.
Here is the video of her speech which starts at the 3:44: 29 mark.