The author - whose seven-book series about the young wizard has sold almost 400 million copies - will not take legal action against George Lippert, who wrote a follow-up to Harry's adventures on his website.
Rowling's lawyers confirmed she is happy for spin-offs to be published online as long as the publications are not sold and it is made clear she was not involved in the stories.
Meanwhile, Rowling - who has seen the Harry Potter books turned into blockbuster movies - has been voted entertainer of the year by Entertainment Weekly magazine.
The publication insisted she deserved the title for getting "people to tote around her big, old-fashioned printed-on-paper books as if they were the hottest new entertainment devices on the planet".
The magazine also named George Clooney a "valedictorian" because he has "deftly balanced box-office viability with personal responsibility".