It's been a long wait for HP fans. Book four, Harry Potter and the Goblet of Fire, was released two-and-a-half years ago.
It became the fastest-selling book of all time, and more than 192 million copies of Potter books have now been sold.
'Brilliant and compelling'
Fans have been waiting for book five ever since.
But JK Rowling never promised when she would finish writing it, and stories that it had been delayed were false.
But now, at last, she is ready to unleash the next instalment of Harry's adventures to the world.
Announcing the news, Nigel Newton, head of Bloomsbury, said: "Harry Potter and the Order of the Phoenix is absolutely superb and will delight all JK Rowling's fans.
"She has written a brilliant and utterly compelling new adventure."
Make sure you keep checking back for all the info as we get it - including where JK's manuscript is now!