More than two million copies of Harry Potter and the Half-Blood Prince were sold in the first 24 hours in the UK.
That's more copies on the first day of sale in the UK than any other book, beating the record set by the last Potter book, the Order of the Phoenix.
The Half-Blood Prince also set a record in the US, where Potter fans snapped up 6.9m copies on its first day of sale.
It's estimated that the new novel has been selling at a rate of 250,000 copies per hour.
The company that counts the number of books sold has already awarded it gold and platinum awards for its sales figures.
"Typically a good book will take about four to six months to go gold and very few books reach platinum in their first year of publication. 'Harry Potter and the Half-Blood Prince' achieved platinum in less than one day!"
The Half-Blood Prince - the sixth in the series - came out in the UK at midnight on Friday.
JK Rowling has told fans the seventh and final instalment of the Harry Potter books could be out in about two years' time.
She was talking at a press conference in Edinburgh, where all the reporters were children aged between eight and 16.
After the press conference, 14-year-old competition winner Owen Jones, from Cardiff, quizzed JK in the only main interview she will give during the Half-Blood Prince's release.
"I think I give very clear pointers as to what Harry will do next," she said of book six.
"There is plenty to guess at... at least one thing I think people will probably deduce, there is a mystery left at the end, but I think they might already know the answer if they think about it."