She made the announcement to thousands of delighted fans from across the globe who came together for a special event celebrating next month’s release of Fantastic Beasts And Where To Find Them.
Speaking at the event in London’s Leicester Square, alongside actor Eddie Redmayne and director David Yates, she said: “We always knew it was going to be more than one movie, and we said a trilogy as a place holder.
“But I’ve done the plotting properly and I’m pretty sure it’s going to be five movies.”
Yates added that the next film will see Redmayne’s character, the animal-loving Newt Scamander, visit another of the world’s major capital cities.
The Fantastic Beasts film, due to hit screens on November 18, is set in New York, and tells the story of a fictional author mentioned in the Harry Potter stories.
Rowling, 51, added: “This is where I was interested in going. This is what I wanted to do.”
The London event, at Cineworld’s Empire IMAX cinema, included a Q&A with cast members Dan Fogler and Katherine Waterston and producer David Heyman, who also brought together all the Harry Potter films.
Actors Jon Voight and Colin Farrell also tuned into the discussion from Los Angeles by video link, which connected to similar events in cities around the world.
Redmayne, 34, revealed he was a huge Harry Potter film fan before he was given the role.
He told the Press Association: “We all grew up with them. They were always like a safe place you could go to every one or two years. Whatever was happening in the world you could go and hang out with Ron and Hermione and the world would be a better place.”
Describing the opportunity to be a part of the Harry Potter legacy as “incredibly exciting and a little bit intimidating”, he added that it would be a while before his four-month-old daughter Iris May would be allowed to watch Fantastic Beasts.
The London-born actor, who won an Oscar and reached international stardom with his portrayal of physicist Stephen Hawking in The Theory Of Everything, revealed that he had a number of things in common with his character Newt, including Hogwarts school houses.
After “sorting” himself on the franchise fan site Pottermore, he said: “Because I’m very methodical, my house is Hufflepuff.
“My Patronus is a Basset Hound. I did it with the guys from Pottermore and they made me do it again and I still got a Basset Hound. They said that was physically impossible.”
When casting the role of socially awkward Newt, Yates said he chose Redmayne for his “unique physicality” and “wonderful shape”.
He said: “He is spindly in a beautiful way and he just felt really soulful. I couldn’t imagine anyone else once I had sat down with him.”
Commenting on working with Rowling, he said: “She has the most extraordinary imagination of our generation and she has created something entirely different. That world is spilling out of her head in a unique way.”