Film studio Warner Bros. announced Thursday that Rowling will write the screenplay for a movie based on "Fantastic Beasts and Where to Find Them," her textbook to the magical universe she created in the boy wizard's stories.
The story will focus on the book's fictitious author, Newt Scamander, and is anticipated to be the first in a series.
Rowling said in a statement the movie "is neither a prequel nor a sequel to the Harry Potter series, but an extension of the wizarding world."
She said the story would begin in New York 70 years before the start of Harry Potter's tale.
Screenwriting is the latest in a string of new ventures for Rowling since she finished writing the Harry Potter series, which has sold more than 450 million copies around the world.
She has published a novel for adults, "The Casual Vacancy," and written detective thriller "The Cuckoo's Calling" under the pseudonym Robert Galbraith.
Rowling said the idea for a "Fantastic Beasts" film had come from Warner Bros., and she soon realized she could not entrust another writer with her creation.
"Having lived for so long in my fictional universe, I feel very protective of it," she said. "I already knew a lot about Newt."
"As I considered Warner's proposal, an idea took shape that I couldn't dislodge. That is how I ended up pitching my own idea for a film to Warner Bros."
As well as movies, Warner Bros. said "Fantastic Beasts" would be "developed across the studio's video game, consumer products and digital initiatives businesses."
Warner Bros., a unit of Time Warner Inc., was behind eight Harry Potter movies, released between 2001 and 2011. Rowling did not write the screenplays for those films, which Warner Bros. says took in $7.7 billion at the global box office.
Since the film series wrapped up, the company has been involved in related ventures including a Harry Potter studio tour near London, Universal's Wizarding World of Harry Potter theme park in Orlando, Florida, and the Pottermore website.
Warner Bros. also said Thursday it would be the worldwide distributor for a television adaptation of "The Casual Vacancy." The BBC is due to film the miniseries next year.