The 30-year-old This Morning presenter made the comments about the bestselling book to Easy Living magazine.
The Surprise Surprise host told the magazine: "I finished the first book and thought, 'This is the best thing I've ever read'."
She continued to reveal that she has read both sequels to the book, Fifty Shades Darker and Fifty Shades Freed.
"I was completely obsessed. Literally couldn't put it down," she explained. "Then I read the second, and the third... I mean, bloody hell, you learn a lot - I think every woman should read them!"
In the same interview, Willoughby also revealed that she gets "pretty sick of talking about [her] boobs".
Fifty Shades of Grey, written by EL James, has sold over 5.3 million copies in Britain, making it the best-selling book since records began.
One Direction singer Harry Styles previously branded the book "educational", and it has been rumoured that Mila Kunis could star in the upcoming film adaptation of the novel.
Photo gallery - Fifty Shades of Grey movie rumoured cast members:
Copyright: PA Images Jonathan Short/AP