The press, poster and TV campaign shows Keaton in a white trouser suit and was shot in an upmarket house in the LA district of Pacific Palisades.
The 60-year-old star joins the likes of Natalie Umbruglia, Eva Longoria, Scarlett Johansson and Penelope Cruz, who have all uttered the memorable strapline: "You're worth it."
This is not the first L'Oreal campaign to feature more mature stars. Last month the cosmetics giant launched a European campaign for Pro-Calcium's sister product, Age Re-Perfect Pro-Calcium, featuring 70-year old actress Jane Fonda. In 2002, 62-year-old French actress Catherine Deneuve was a face of the company.
Carol J. Hamilton, president of L'Oreal Paris in the US, said: "We have had an incredible response from so many women to the news that L'Oreal signed Diane Keaton as our spokesperson.
"Diane is an exemplary role model for women in every stage of their lives, and especially for women of her generation. She possesses such a luminous beauty and personality, and she is incredibly smart and truly cares about how women feel about themselves. We're thrilled that she'll be starring in our newest L'Oreal ads."
In the last two years, L'Oreal spent more than $10 million on media in the US for its Age-Perfect line. The cost of the Keaton campaign was not revealed by L'Oreal.
Interpublic Group's McCann has handled L'Oreal's business in the US since 1973. The agency also handles the company's hair-care and hair colour brands. Publicis is also on the advertiser's global agency roster.
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