Welcoming her second arrival with her former Emmerdale star husband James Thornton, Joanna wrote: "So happy! Kit James our baby boy was born yesterday at 4:14pm weighing 8lbs! He's beautiful!"
James and Joanna are already parents to two-year-old daughter Eva.
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Joanna Page said that motherhood has made her more "responsible"
While the Love Actually actress may be an only child, Joanna has made no secret of her desire to expand her brood. The Welsh beauty previously admitted that she would like to have as many kids as possible.
"James is one of four so to him, four children is the norm," she told WalesOnline. "For me, one is the norm but I can see how people get addicted to having babies. 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 37-year-old added that being a mother has changed how she acts in front of her brood, and has made her want to set the best example possible. It's meant less indulging in cake and switching off the radio and TV if something explicit comes on.
James Thornton and Joanna Page are already parents to daughter Eva
"Having a child changes the way you are," said Joanna. "So many more things are important because they learn from what they watch.
"I know I sound like a really boring mum but every song you hear on the radio is about sex or not respecting yourself. There might be a song that I love to sing myself but I can't play it in front of Eva. She's brought out my responsible side."
The RADA graduate has starred in films including From Hell and Mine All Mine, but it was her leading role in BAFTA-winning television series Gavin & Stacey, which she featured in from 2007 to 2010, that propelled Joanna into the limelight.