Joanna shared the joyous news with her fans on Twitter, revealing her son's sweet name. "Our beautiful baby boy Noah Wilder Russell was born Tues the 13th at 8:51am, weighing 7 pounds 3. Welcome to our gang Noah! We love you xxx," she wrote.
James, who plays John Barton on Emmerdale, also confirmed the news, tweeting: "Christmas has come early for us! Baby number 3 is here! Our beautiful baby Noah arrived Tues 13th 8.51am 7lbs3. He is amazing X."
Joanna Page and husband James Thornton have named their son Noah
Joanna, 38, and her husband James, 41, are also the proud parents to Eva, three, and Kit, who turns two in spring.
The actress has made no secret of her desire to have a large family, saying: "James is one of four so to him, four children is the norm. For me, one is the norm but I can see how people get addicted to having babies."
She added to Wales Online: "I would like to keep going and try to pop out as many as possible – I only wish I'd started having them younger."
The couple are also parents to Eva and Kit
The Love Actually star announced her pregnancy in June, joking that her morning sickness had been a "breeze" compared to raising her two children. "Some good news! We've got baby number 3 on the way!!!! ... Morning sickness is a breeze when you're already looking after a 3 and 1 year old!" she tweeted.
James and Joanna both appeared in the 1999 TV drama David Copperfield. They were reintroduced a year later through mutual friends and went on to marry in December 2003.