The 41-year-old French actress gave birth to her little one with actor-director beau Guillaume Canet, her rep confirmed to People magazine.
Last September, Cotillard had revealed on Instagram that she and Canet were expecting their second child together.
She then wrote, "Many years ago, I met the man of my life, father of our son and of the baby we are expecting. He is my love, my best friend, the only one that I need."
The couple is already parents to a boy named Marcel.
Cotillard and Canet first starred together while shooting for the 2003 film "Love Me If You Dare" and began dating in 2007.