The writer told fans on her website that she's working on a collection of material relating to the boy wizard's world, and informed them the proceeds will be given to her favourite organisations.
"For a long time I have been promising an encyclopedia of Harry's world, and I have started work on this now," she writes.
"It is likely to be a time-consuming job, but when finished I shall donate all royalties to charity."
Rowling also confirmed she has no current plans to write another Potter book.
"I have always refused to say 'never' to this question, because I think it would be foolish to rule out something I might want to do in a few years' time.
"However, I have no immediate plans to write another Harry Potter novel, and I do think that I have rounded off Harry's story in the seven published books."