He will take over from Jeff Stelling and join regulars Rachel Riley and Susie Dent when the new series of the maths and words quiz begins next year.
Nick said: "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.
"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 spell chrysanthemum."
Nick will become the fifth presenter of what is Channel 4's longest-running show when he takes over.
Jeff announced earlier this year that he was to leave after less than three years in the hotseat to concentrate on his presenting role at Sky Sports.
The longest-serving host was Richard Whiteley, who launched the show at the birth of the channel in November 1982 and continued until his sudden death in 2005.
Channel 4 chief creative officer Jay Hunt said: "Nick will be a fantastically charismatic host for Countdown. He is sharp and funny and has a real twinkle in his eye."