Rowling 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 since a mysterious "Pottermore" website appeared last week.
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 new material that she has written on the backgrounds to the characters. It will go live from July 31 for one million fans who pass a special "test" and to the general public from October.
"It's a great way to get back to the Harry Potter readership," she told reporters. "I had more than half of the new material already written."
But she revealed that she had "no plans" to write another novel.
The seven e-books will be available through the website from October in partnership with Sony.
