Called Harry Potter and the Cursed Child, it will tell the “untold part” of the boy wizard’s story, including the story of the lives of his murdered parents.
However, the author said on Twitter the play was “not a prequel”.
It will run at the Palace Theatre in the summer of 2016, with tickets on sale this autumn.
Rowling said she chose to make the announcement on Friday because it was “a very special day” – the 18th anniversary of when her debut book, Harry Potter and the Philosopher’s Stone, was first published in the UK.
But she was tight-lipped about the plot of the play, saying she did not “want to say too much more, because I don’t want to spoil what I know will be a real treat for fans”.
She added: “To answer one inevitable (and reasonable!) question – why isn’t Cursed Child a new novel? I am confident that when audiences see the play they will agree that it was the only proper medium for the story.”
The play was first announced in December 2013, after the author said she had “received countless approaches” over the years “about turning Harry Potter into a theatrical production”.
Although the new story has not been written by Rowling, it is a collaboration between herself and writer Jack Thorne – who previously adapted Let The Right One In for the stage and won a Bafta for his work on Channel 4’s This is England ’88 – and John Tiffany, who will also direct the play.