Channel 4 confirmed that Lord Alan Sugar's aide will take over from Jeff Stelling when filming of the long-running gameshow recommences at the end of the year.
Hewer's first episode as Countdown host will air on January 9, 2012.
"I'm flattered to have been asked to present such an iconic programme and will work hard to justify sitting in the same seat as such great talents as Jeff Stelling, Des O'Connor and of course the great Richard Whiteley," Hewer commented.
"It's particularly fitting that I should be doing this now as my spelling has started to slip quite badly. I used to be able to spell chrysanthemum."
Channel 4 chief creative officer Jay Hunt said of the announcement: "Nick will be a fantastically charismatic host for Countdown. He is sharp and funny and has a real twinkle in his eye."
Head of daytime Helen Warner predicted that Hewer would bring "dry wit, fierce intelligence and unique charm" to the show, while executive producer Peter Gwyn added: "I'm absolutely delighted that Nick has agreed to take the Countdown chair, and I can't wait to see him bring his unique style and humour to this iconic format.
"With Nick at the helm and our 30th birthday next year, we've got some very exciting times ahead for Countdown viewers."
Maths expert Rachel Riley and Dictionary Corner chair Susie Dent are both returning to the show.
Stelling quit as Countdown presenter back in May after two years in the role.
Giles Brandreth and Rory Bremner were among the stars tipped to succeed him.