The Ickabog, which is a fairy tale for children, will be made available for free online for kids in lockdown or back at school, she said.
The Harry Potter author said she wrote the story 10 years ago, but stord it away in a box in the attic as she published her novels for adults.
Rowling said she has now retrieved it and done some re-writing and will be publishing a chapter online every weekday.
Sharing the news on Twitter, she wrote: "I have a small announcement, but before I get started, I want to head off one possible source of confusion. THIS IS NOT A HARRY POTTER SPIN-OFF.
"Over 10 years ago, I wrote a stand-alone fairy tale called The Ickabog."
She added: "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.
"Until very recently, the only people who'd heard the story of The Ickabog were my two younger children.
"Over time I came to think of The Ickabog as just for my family. The manuscript went up into the attic, where it remained until a few weeks ago."
Sharing a photo of the book she kept the pages on, she added: "This is the very dusty box I got down from the attic. (It's a Net-A-Porter box and might well have held a premiere dress.)
"Opening the box was like opening a time capsule. Most of the story was handwritten, but bits had been typed up. When I put it into some kind of order (I'm not renowned for my filing skills) I had a patchwork first draft."
She continued: "Anyway I've decided to publish The Ickabog for free online, so children on lockdown, or even those back at school during these strange, unsettling times, can read it or have it read to them.
"We'll be publishing a chapter (or 2 or 3) every weekday, starting at 3.00pm today and ending on July 10th. You don't need to register to access the story. You can read it for free on a new website we'll be launching shortly. But there's more...
"I'd like children to illustrate the book for me! I'll be suggesting ideas for pictures as we go, but nobody should feel constrained by my ideas. Let your imaginations run wild!"
Rowling said best pictures in each publishing territory will be included in the books she intends to publish in November 2020.
She added she will not be judging the competition but encouraged parents or guardians to share the picture on social media.
She continued: "Lastly, I intend to donate my author royalties from the published books to projects and organisations helping the groups most impacted by Covid-19. Full details will be made available later in the year.
"The first two chapters of The Ickabog will be going live in a couple of hours time. Whether you read it yourself, or have it read to you, I hope you enjoy it."
Earlier this month Rowling announced she was donating £1 million to help the homeless and those affected by domestic abuse during the coronavirus pandemic.
Announcing the financial contribution - which will be split between the Crisis and Refuge charities - Rowling said the money would go towards helping some of those hit hardest by the outbreak and the impact of the lockdown.
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