Some Potter fan sites, which have been instrumental in generating a large and loyal fan base for the movies and seven-book series on which they are based, were given a sneak preview of the mysterious new website.
Rowling and Potter studio Warner Bros. have never shied away from building up the hype ahead of key releases in the series.
The Harry Potter novels have sold more than 400 million copies worldwide, while the seven movies released so far have grossed some $6.4 billion in ticket sales. Rowling has been billed the "world's first billionaire author."
A spokeswoman for Rowling confirmed that the site was genuine.