Lauren Weisberger's new story, titled Revenge Wears Prada: The Devil Returns, will continue the adventures of former magazine assistant Andy Sachs.
Set eight years after the first book, the story will see Andy - now a bridal magazine editor - crossing paths with her old boss, Miranda Priestly, again.
The book will be published by HarperCollins in the UK next April.
It had been under contract as part of a multi-book deal, but it was not known if Weisberger would ever write a sequel.
The sequel will also see the return of Andy's Runway magazine catty rival Emily, who now works alongside her at The Plunge and has become her best friend.
HarperFiction publishing director Lynne Drew said: "I'm thrilled to be once more encountering the world of glossy mags, Miranda Priestly and our favourite heroine, with a novel set to be one of the big summer reads of 2013."
The Devil Wears Prada was a hit when it was first released and spent more than a year on the New York Times bestseller list.
It was adapted into a Hollywood film in 2006 starring Anne Hathaway as aspiring journalist Andy and grossed more than $300m (£196m) worldwide.
Meryl Streep was Oscar-nominated for her role as the overbearing Miranda - widely considered to be a fictionalised version of Anna Wintour, Vogue's editor-in-chief, where Weisberger was once a former assistant.
The author of The Devil Wears Prada has confirmed she is writing a sequel to the best-selling 2003 book.