The multi-million-selling British author made the announcement at a press conference in London after days of fevered speculation about her new project, which had been shrouded in secrecy and rumour.
The free website, www.pottermore.com, will go live from July 31 for one million Potter fans who pass a special online challenge, and to the general public from October.
The seven e-books will be available through the website from October in partnership with Sony.
"It's a great way to get back to the Harry Potter readership," Rowling told reporters.
She said Potter fans will be able to register on the free website using one of the young sorcerers from the books as their online identity, then play games and interact with elements of the fictional world.
The site will also have previously unpublished material that she has written on the backgrounds to the characters and their lives at Hogwarts Academy, a fictional school for young wizards.
"I had more than half of the new material already written," Rowling said, adding that some of it had been "literally" dug out of boxes.
"I wanted to give something back to the fans that have followed Harry so devotedly over the years, and to bring the stories to a new digital generation," she said.
"I hope fans and those new to Harry will have as much fun helping to shape Pottermore as I have."
Rowling revealed that she had "no plans" to write another Harry Potter novel but said that a rumoured Potter encyclopaedia was a possibility.
The author laid down her pen -- and Harry's magic wand -- when she finished the seventh book in 2007, with a stunning record of 400 million copies of the series sold around the world.
She said, however, that she was unable to leave Harry Potter and his world behind.
"It is exactly like an ex-boyfriend," she said.
"I only cried like that when my mum died. It completely changed my life. It is impossible to overstate what Harry Potter meant to me. I adore my readers."
Next month sees the release of "Harry Potter and the Deathly Hallows: Part 2," the final film in the epic saga, starring Daniel Radcliffe as Harry, Emma Watson as Hermione Granger and Rupert Grint as Ron Weasley (.AFP)
