Rowling broke the news on Twitter, telling fans the book is not a spin-off of her best-selling "Harry Potter" series, but a brand new story.
Rowling said in a press release on her website that she wrote "most of a first draft" more than 10 years ago, while she was still writing the "Harry Potter" books.
"I always meant to publish it, but after the last Potter was released I wrote two novels for adults and, after some dithering, decided to put those out next," Rowling tweeted, adding, "until very recently, the only people who'd heard the story of The Ickabog were my two younger children."!
At least one chapter of the story will be published every weekday from Tuesday until July 10 on "The Ickabog" website, she said.
The story will be officially published in English in November, with the "best drawings in each territory" included in the copies, Rowling's website said.
When the story is published and available for sale, Rowling said she will donate her earnings