The Kenyan star, who shot to fame in the film 12 Years a Slave, is Lancome's first black ambassador.
It is the 31-year-old's first major endorsement, and she joins Julia Roberts, Kate Winslet, and Penelope Cruz as promoters of the brand.
Lancome advertisements featuring Ms Nyong'o are due start appearing this summer.
The actress said in a statement released by the French company: "I am truly honoured to join the Maison Lancôme, a brand with such a prestigious history that I have always loved.
"I am particularly proud to represent its unique vision for women and the idea that beauty should not be dictated, but should instead be an expression of a woman's freedom to be herself."
She received an Academy Award this year in the category of Best Supporting Actress for her portrayal of Patsey in the Steve McQueen-directed film.
She is the fourth Academy Award-winning Lancome ambassador.
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