The Mad Men beauty, a former model, confirmed the news by posting a photo of her posing by a billboard on the streets of Paris, France.
She added the caption: "Found this chick in Paris, she's cute eh? Announcing my partnership with @kerastase_official #nutritive."
The South Dakota native has been a style icon and designer's dream for years and struck a deal reportedly worth millions with Versace in 2009. Ever since, the single mum has stunned on red carpets in the latest Versace fashions, including the red gown she wore to the 2011 Golden Globe Awards in Los Angeles.
And she's not the only celebrity fan of Kerastase - Kate Hudson's hooked on the salon brand's products, revealing on Instagram last year she uses Kerastase's shampoo, moisturiser and eye cream to look shiny and fresh. She kicks off her beauty regime by shampooing with the Kerastase Nutritive Bain Nutri-Thermique, developed especially for dry hair. Then she gives her luscious locks a moisture boost with the Kerastase Chronologiste Essential Revitalising Balm.
January joins model and fashionista Anja Rubik as an ambassador for the beauty brand - the Polish stunner, who was recently ranked the ninth richest model in the world, was named the new face of Kerastase’s Visions of Style II campaign in December.
Rubik, who has walked the runway for Balmain and Chanel, is also editor-in-chief of 25, a magazine she founded.
She helped the firm launch L'Incroyable Blowdry, which will debut in salons next month, with the help of in-demand stylist Luigi Murenu.