The actress is the new celebrity face of the haircare part of the beauty brand.
Dunst commented on her new role: "I feel very close to L'Oréal Professionnel.
"The brand is strongly connected to fashion and I admire its commitment to innovation, constantly creating new professional services and products to achieve the most on-trend styles."
L'Oréal Professionnel's International managing director Anne-Laure Lecerf said: "Kirsten Dunst has a timeless beauty and is a modern woman, closely connected to fashion. She has a strong personality and makes her own choices.
"Her great professionalism and commitment to excellence make Kirsten one of the most talented actresses I have ever met.
"She has the perfect attributes to embody the new values of L'Oréal Professionnel."
Although the On the Road star is the first global ambassador for the haircare brand, numerous famous faces have modelled for the company's cosmetics section, L'Oréal Paris, including Cheryl Cole, Julianne Moore, Lea Michele and Jennifer Lopez.