The Oscar-winning actress used Instagram to share a flawless picture of herself in the ad.
"The secret I never intended to keep," she captioned the snap, which now has over 66,000 likes.
Lupita's contract with the French beauty brand was announced back in April. She is the first ever African woman to represent the cosmetics giant.
She follows in the footsteps of fellow Hollywood stars Kate Winslet, Julia Roberts, Emma Watson and Penelope Cruz.
Lupita stars in the ads for Lancome's Tient Idole 24H foundation, which is available in 28 shades.
The stunning actress was announced as the face of Lancôme back in April
"I am truly honoured to join the Maison Lancôme, a brand with such a prestigious history and that I have always loved," said the 12 Years a Slave star in a statement.
"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."
Lancôme boss Françoise Lehmann added: "Both talented and committed, true to her African beauty, and showing a great curiosity and open-mindedness in her career choices, Lupita is by essence the Lancôme woman.
"A strikingly beautiful and intelligent woman."