Edith Vivian Patricia Upton is the fourth child for the Oscar winner, who is already mom to Ignatius Martin, 6, Roman Robert, 10, and Dashiell John, 13, with her husband, whom she married in 1997.
"The Upton family (Cate Blanchett and [husband] Andrew Upton) have adopted a baby girl," her rep told ABC News.
"When I was approached to be in this, I said, 'It wasn't a fairy tale that I actually gravitated towards,'" she said. "Because I found the central female character really passive."
Blanchett, 45, eventually singed on and say playing a villain was "fantastic."