Hewer will replace Sky Sports presenter Jeff Stelling as the host of the long-running daytime series in the New Year.
The former PR consultant follows in the footsteps of previous hosts Des O'Connor, Des Lynam and Richard Whiteley.
Maths expert Rachel Riley and Dictionary Corner chair Susie Dent are both returning to the show.
Speaking about his appointment earlier this year, Hewer 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 to spell chrysanthemum."
Nick Hewer's first Countdown episodes will air in January 2012 on Channel 4.