In her new role she is working with the global brand on campaigns to celebrate friendship, travel, fashion and fun and will lend her voice to "an exciting programme" of promotional activity.
The This Morning host, who has also endorsed brands like Garnier, Oral-B, Very and BHS, is set to increase her fortune further with this new venture.
The former model joins the likes of pop star Taylor Swift as a face and brand ambassador of Diet Coke.
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Aedamar Howlett, Marketing Director for Coca-Cola Great Britain describes Holly as "the perfect partner" for the brand and says they're "proud to be working with her to bring some extra sparkle to Diet Coke activity this year."
Speaking about the new endorsement partnership, Holly said, "I’ve always been a big Diet Coke fan, right from the original ‘Break’ ads we all remember with great fondness.
“The brand is well known for celebrating friendship, fashion and fun, which are all very important to me. I’m excited to be involved!"
And whilst presenting now comes naturally to the 36-year-old television presenter, Holly previously admitted she was a "c**p" TV presenter when she first started out.
She said: "I've been really c**p at my job. In the beginning I was terrible, although I enjoyed doing it, which was kind of more upsetting. I spoke in a posh telephone voice, and I was so unnatural: I fixated on remembering lines rather than just speaking. It took me two years working away from the camera in a TV studio until I went back to presenting."