Cotillard paired a plain white maxi dress with a simple gold neck chain and ruffled jeans from the fall 2017 collection of Y/Project by Belgian designer Glenn Martens.
She finished her look with a pair of strappy stilettos.
The 41-year-old actress, who in past years has rocked attires by Alexander McQueen and Maison Martin Margiela, sported the surprisingly dressed-down look during a photo call for her upcoming film "Ismael's Ghosts".
She posted a picture of her wearing the creation and was clicked by award-winning photographer Eliott Bliss.