The 44-year-old shared an image to social media of her cradling her daughter and announced she had been named Boe Willow Russell.
Boe is Joanna’s fourth child with fellow actor, Thornton, best known for his role as John Barton in Emmerdale.
Joanna Page welcomes fourth child
Page, who played Stacey Shipman in the Bafta-winning TV series Gavin & Stacey, posted a photo to Instagram which shows her holding her newborn daughter beside a Christmas tree.
She captioned the post: “Welcome to our family Boe Willow Russell. We love you with all our hearts @jamesthornton_”.
Thornton reshared the same image and added: “She is here! Our wonderful girl Boe Willow Russell.”
Celebrities congratulate Joanna Page and James Thornton on birth of daughter
The couple have a daughter Eva, who was born in February 2013, and sons Kit born in April 2015 and Noah born in December 2016.
Friends and famous faces offered their congratulations, including fellow Emmerdale star Charley Webb who commented: “The loveliest news and the loveliest name.”
TV host Dermot O’Leary wrote “The best news! Congrats. Xx” while actress and presenter Denise Van Outen added “Congratulations” with a heart emoji.