It is lovely news for a lovely couple, and The Weekly is thrilled to share their news, which their management has confirmed with us.
For the past two days, the Oscar-winning actress has been arriving to collect her sons from their primary school in Sydney, snuggling the newborn to her chest.
Cate, 45, has told friends at the local school that she and her husband, theatre producer Andrew Upton, 49, are deliriously happy with the new addition to their family. The little girl is believed to be called Vivian for short.
"She's really excited and clearly besotted," said the parent.
The couple already have three children – all boys – Dashiell, 13, Roman, 10 and Ignatius, six. Cate has spoken previously in interviews about her desire for a fourth child.
The Weekly contacted Cate's representatives who have confirmed the parents have adopted a baby girl.
The family have recently announced they will be relocating to the United States, after her husband's term as artistic director at the Sydney Theatre Company ends in 2015. But Australia is close to their hearts, so they have vowed to return here at a later point.
Cate clearly treasures her family and is regularly seen on the school run, collecting her two youngest boys from primary school, while her oldest is already at high school.
Life with three boys has at times left Cate feeling "a bit demented" as she told W magazine.
But at the time she was quick to add that "the chaos of it is great." So great, in fact, the couple weren't ruling out a fourth child.
"Everyone says, 'Oh, you must have [been trying] to have a girl,'" Cate says. "If the next one was a boy, then that's just our lot!"
Adopting a baby has proved great joy to many high-profile couples, including Angelina Jolieand Brad Pitt, Deborra-lee Furness and Hugh Jackman and Madonna.
Recently, the Federal Government announced plans to examine adoption laws in Australia to make it easier for families to adopt babies from both here and overseas.