The 48-year-old author made the announcement at the Harrogate Crime Writing Festival in England over the weekend.
The crime novel series focuses on detective Cormoran Strike, and is written under the pseudonym Robert Galbraith. Strike is a former military police investigator who first appeared in The Cuckoo's Calling in 2013, and The Silkworm in June this year. Rowling said the series is "pretty open ended" in regard to sequels, and will eventually outnumber her seven Harry Potter books.
"I really love writing these books, so I don't know that I've got an end point in mind," she shared. "One of the things I absolutely love about this genre is that, unlike Harry, where there was an overarching story, a beginning and an end, you're talking about discrete stories. So while a detective lives, you can keep giving him cases."
Rowling said she is "about halfway" done with the third novel, which will focus on "what happens to people after they leave the military." She already has ideas for the fourth book, and is also busy writing her first film script for Harry Potter spinoff Fantastic Beasts and Where to Find Them.