Agent Christopher Little said the series reached the milestone following the publication of "Harry Potter and the Half-Blood Prince," the sixth volume about the schoolboy wizard.
Potter books have now been translated into 63 languages, most recently Farsi.
"Half-Blood Prince" sold an astonishing 6.9 million copies in the U.S. in its first 24 hours on bookshelves when it was released in July, averaging better than 250,000 sales per hour and smashing the U.S. record held by the previous Potter release.
Rowling also tops the list of Amazon.com's best-selling authors.
The online retailer, celebrating its 10th anniversary, announced its biggest sellers in July, with Rowling no surprise at No. 1. The "Harry Potter" author was followed by Spencer Johnson of "Who Moved My Cheese?" fame, romance novelist Nora Roberts and "The Da Vinci Code" author Dan Brown.
William Shakespeare was No. 26, one notch below Tom Clancy.