The EastEnders favourite welcomed the little bundle of joy with her fiancé Marc Humphreys over the bank holiday weekend.
The 33-year-old actress shared a sweet photograph of the new addition to her family, which includes her five-year-old daughter Eliza from her ex-partner Adam Cottrell.
Natalie captioned the heart-warming photograph taken in hospital: “@cameramarc and I introduce Joanie Elizabeth Cassidy-Humphreys… We are all so in love!”
Natalie and Marc got engaged in October last year and the TV star - who plays Sonia Fowler in the BBC One show - revealed she was expecting her second child in February.
She said: "Evening all… Time to let everybody know that @cameramarc and I are having a baby in the summer. The three musketeers become four.”
Meanwhile, the happy couple’s friends and family have warmed to Marc and can’t wait for the couple to walk down the aisle in the near future.
A source said: “Family and friends are thrilled for Natalie and have been buying gifts for them to celebrate.”
Natalie’s ex Adam was charged with assaulting her and though she got back with him for the sake of their little girl, it didn’t work out.
She said at the time: “It was my decision. I’m really glad I gave it another chance. When kids are involved you almost have to. I can always say to Eliza, ‘Mummy tried, but it didn’t work out.’”