The actress, 33, took to Twitter to announce the birth of the little one, telling her 313,000 followers, '@cameramarc and I introduce Joanie Elizabeth Cassidy-Humphreys... We are all so in love!'
The tweet was accompanied by an adorable family picture of Natalie, her fiancé Marc, newborn Joanie, and 5-year-old Eliza, Natalie's daughter from her previous relationship with partner Adam Cotrell.
Natalie, who is best known for her role as Sonia Fowler in the BBC soap, also used Twitter to confirm her pregnancy in February this year.
'Evening all...', she wrote. 'Time to let everybody know that @cameramarc and I are having a baby in the summer. The three musketeers become four.'
Evening all... Time to let everybody know that @cameramarc and I are having a baby in the summer. The three musketeers become four.— Natalie Cassidy (@Nat_Cassidy) February 12, 2016
Congratulations from fellow celebs were quick to pour in, with Jacqueline Jossa replying 'Ooooo gooodnessss I'm so happy for you both!! Amazing news!! ❤️❤️ xx' and Nicola McLean agreeing 'so happy for you both 😍😍😍😍😍👶🏼 xxx'.
Marc and Natalie became engaged in October 2015, with a spokesperson for the soap star saying, 'She is really happy.'
In an interview with the Mirror in February of the same year, Natalie opened up about her hopes for her daughter, then four, explaining that she wouldn't want her to follow in her footsteps as a child actress.
'I wouldn’t want her to do it because I don’t know what it’s like to not be recognised,' she said.
'It's not a bad thing, because I'm so used to it, but I'd like her to retain her anonymity. I think it’s important. I’d like her to have that for her life, really. I don’t want her to have what I’ve had.'
However, despite her concerns for Eliza's privacy, she added that her family situation at the time made her 'really happy'.
'All I know is that I’m really happy,' she concluded. 'I am the most content I've ever been. It's brilliant. I'm in a lovely place right now. Work's good, life's good, family are good. It’s all good.'