Britvic has signed the reality star, who is married to musician Professor Green, to appear in a series of photos aimed at promoting the new drink.
Mackintosh will appear in photos that are said to represent the sparking nature of Spritz, including one in which she is surrounded by bubbles.
The brand is also rolling out social media activity with the hashtag #BlendRecommends, with the celebrity amplifying the brand through her own social media channels by posting her "favourite blends". The campaign will be Instagram-led.
"We’re really excited about the launch of J2O Spritz and who better to work with than Millie Mackintosh to help bring blends to life?" said brand marketing director, Annabelle Cordelli.
"We’re really looking forward to hearing what people think of not just Spritz, but also the #BlendRecommends campaign."
J2O Spritz is available in three variants: Pear and Raspberry, Apple and Watermelon and Peach and Apricot.