The Saturday Night Takeaway hosts shared the exciting news on their Instagram account on Wednesday, April 1.
Ant and Dec shared a video which showed them separately from their houses.
In the clip, Dec said: "Bit of an announcement today but rather than just doing it straight, I thought we could have a bit of fun with it by playing a game of charades."
Dec then did an action, as Ant shouted: "A book!"
Dec said: "Very good," as she started to do other actions.
After multiple guesses, Ant couldn't get it right and Dec said: "Forget it.
"What I was trying to say was this year, me and Ant would have been celebrating 30 years working together.
"So, we've written a book. Here it is. It's out on the 3rd of September."
Ant said: "You can't talk during charades," to which Dec told him: "Shut up!"
The caption read: "A bit of news from our lockdowns to yours… this year we’ll have been working together for 30 years (blimey!) and we’re writing a book to mark the anniversary.
"Stories from our time on the telly (and that brief period of time when we were basically 90s pop sensations!).
"Excited for you to have a flick through… less excited for you to see the photos we’re including, but we accidentally promised we’d put them in and there’s no backing out of that now!
"Sending you all lots of love during these strange times."
Fans were thrilled by the news and congratulated the pair.
What did their fans say?
One person commented: "AHHHH!!! I'M SO EXCITED!"
Another wrote: "Pre-ordered - can’t wait," while a third added: "YES!!!!!!! Early birthday present for me."
Ant and Dec were recently praised by their fans for providing entertainment on Saturday Night Takeaway last weekend.
The pair hosted from their houses after a three-week lockdown was enforced in the UK amid the coronavirus pandemic.
One person said on Twitter: "Started crying as soon as #SaturdayNightTakeaway came on today. Sad and happy tears."
In addition, another wrote: "That was very different and a first for @itvtakeaway and so proud to be a part of it.
"So proud of you @antanddec for keeping the nation smiling."
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