She said: "The freedom to explore new territory is a gift that Harry's success has brought me, and with that new territory it seemed a logical progression to have a new publisher.
"I am delighted to have a second publishing home in Little, Brown, and a publishing team that will be a great partner in this new phase of my writing life."
Rowling’s agent Neil Blair has left the Christopher Little Literary Agency, creating The Blair Partnership and taking his famous client with him.
A statement from Rowling’s PR representatives, Stonehill Salt, confirmed the author had “terminated her association with the Christopher Little Literary Agency".
Details about the book are a closely-guarded secret and its title and publication date will be announced later this year.
Crime writer Ian Rankin, who like Rowling is based in Edinburgh, teased fans on Twitter by hinting at a possible plot for the new book.
He said: "Wouldn't it be funny if JK Rowling's first novel for adults turned out to be a crime story set in Edinburgh? My word yes."
The book is guaranteed to be a best-seller, with Rowling's fans around the world eager to see what she does after Harry Potter.
The writer, whose personal wealth was estimated at £530 million in the latest Sunday Times Rich List, spoke about how much she missed her most famous character last year when she unveiled her website, Pottermore.
She admitted that working on the project, which has been in development for several years, allowed her to "stay in touch" with the period in her life when she was writing the stories.
She said: "It is true to say that finishing writing Harry, I have only cried that much, ever in my life, when my mum died.
"I've never cried for a man like I've cried for Harry Potter."
Twitter reaction to the news
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