The film, called 'Fantastic Beasts And Where To Find Them', will be based on a minor character in her stories about the teenage wizard.
It is the first in a planned series inspired by the textbook of the same name given to pupils at Hogwarts school, and will feature its fictitious author, Newt Scamander.
Writing on her Facebook page, Rowling said: "It all started when Warner Bros came to me with the suggestion of turning Fantastic Beasts And Where To Find Them into a film.
"I thought it was a fun idea, but the idea of seeing Newt Scamander, the supposed author of 'Fantastic Beasts', realised by another writer was difficult. Having lived for so long in my fictional universe, I feel very protective of it and I already knew a lot about Newt.
"As hardcore 'Harry Potter' fans will know, I liked him so much that I even married his grandson, Rolf, to one of my favourite characters from the Harry Potter series, Luna Lovegood.
"As I considered Warners' 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.
"Although it will be set in the worldwide community of witches and wizards where I was so happy for 17 years, 'Fantastic Beasts And Where To Find Them' is neither a prequel nor a sequel to the Harry Potter series, but an extension of the wizarding world. The laws and customs of the hidden magical society will be familiar to anyone who has read the Harry Potter books or seen the films, but Newt's story will start in New York, 70 years before Harry's gets under way."
The commercial success of the 'Harry Potter' books and films, which made stars of Daniel Radcliffe and Emma Watson, guarantees the new film will be a hit. The last film in the series, 'Harry Potter and the Deathly Hallows - Part 2', took £23 million in the UK and Ireland in its opening weekend alone.
Earlier this year, Rowling was unmasked as the author of a crime novel, 'The Cuckoo's Calling', written and published under the pseudonym of Robert Galbraith. The book, which sold about 8,500 copies before she was named as the author, has since become a worldwide best-seller.