Knightley, who has been named sexiest woman in the world by FHM magazine today, has been chosen to front Chanel's Coco Mademoiselle scent campaign.
She follows in the footsteps of Nicole Kidman, who starred in a spot for Chanel No 5 and Selma Blair who was signed in December to promote the brand's glasses.
Kate Moss has also represented the cosmetics giant as the face of Coco Mademoiselle fragrance, but she was dumped after the cocaine scandal broke last October.
The announcement confirms two months of speculation that the star was in talks with Chanel. She already has a 拢1m contract to promote Royal jeweller Asprey.
A spokesperson for Chanel said: "Keira's elegance, beauty and modernity are but a few of the qualities that parallel the iconic faces featured in the legendary Chanel fragrance campaigns, including Mademoiselle Chanel as the first face of No 5 in 1937, followed by Catherine Deneuve and most recently Nicole Kidman."
Jacques Helleu, Chanel artistic director, said: "Keira Knightley is a bright, young actress who has already made her mark in a diverse portfolio of films, including her Oscar-nominated performance in 'Pride and Prejudice'."
Knightley tops a poll of FHM magazine's world's 100 sexiest women after beating of competition from Halle Berry, Charlize Theron and Scarlett Johansson.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .