The 38-year-old announced the news on her Twitter page on Thursday 15 December, sharing her newborn son’s name, weight and birthday.
“Our beautiful baby boy Noah Wilder Russell was born Tues the 13th at 8:51am, weighing 7 pounds 3,” she wrote.
“Welcome to our gang Noah! We love you.”
Thornton also shared the news on Twitter, writing: “Christmas has come early for us! Baby number 3 is here.”
The couple, who are also parents to Eva, three, and Kit, 20 months, announced they were expecting their third child in June 2016.
“Some good news! We’ve got baby number three on the way,” Page tweeted at the time. “Morning sickness is a breeze when you’re already looking after a three- and one-year-old.”