Brydon will appear in a TV ad, created by Leo Burnett, in the guise of comedy newsreader Bruce Bowls.
The campaign, which Kellogg said is worth £13m, kicks off today (Monday) and will also run across print, outdoor, radio and digital mediums. Media buying and planning is by Carat.
Activity will run for an indefinite period and comes as Kellogg looks to fight off competition from own-label and other competitors.
The campaign coincides with new-look packaging for Crunchy Nut and its variants - which includes Crunchy Nuts Bites and Crunchy Nut Nutts - as Kellogg looks to give the brand better standout on the shelves.
Brydon said: "I'm delighted to be a part of such a popular brand and am looking forward to bringing my new character, Bruce Bowls, to life."
Jonathan Ross, Ross Kemp, Lisa Snowdon, Ian Wright, Dame Kelly Holmes and Sir Chris Hoy are among the celebrities who have endorsed the company's cereals in the past.
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