The Twilight actress will be a spokesperson for the women's scent when it debuts this autumn, according to WWD.
Stewart said of the endorsement: "Balenciaga has always stood out to me. Even when I first began to consider fashion and admittedly rarely knew the brand I was wearing, I always recognised and loved Balenciaga. It feels full circle to be involved with something that spoke to me then and of course continues to excite me now.
"The brand is just cool. That's the way it is. I feel drawn to it. It provokes on a number of levels. It isn't afraid or contrived. It hits a very cool balance, always. Always pretty undeniable."
Balenciaga's creative director Nicolas Ghesquière told the website that Stewart is the perfect choice for the new campaign because of her cutting-edge fashion sense.
"Kristen embodies the modernity of the new Balenciaga fragrance with her unique sensibility and intelligence," Ghesquière said.
Balenciaga was launched nearly a century ago in 1914 and has become one of the pre-eminent French fashion houses.
Stewart will next star alongside Charlize Theron and Chris Hemsworth in Snow White and the Huntsman, opening on June 1 in the US and UK.
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Watch Kristen Stewart on the Snow White and the Huntsman flashback trailer below: