Natalie, who plays Sonia Fowler in the BBC One soap, welcomed the tot into the world with her fiancé Marc Humpreys and they are besotted.
The couple have called their daughter Joanie Elizabeth and they have shared a family picture with fans online.
The snap shot shows Natalie cradling Joanie in her arms with Marc and Natalie's daughter from her first marriage, Eliza, looking on.
Posting it on Twitter, Natalie said: "@cameramarc and I introduce Joanie Elizabeth Cassidy-Humphreys... We are all so in love!"
In March, Natalie confirmed she and cameraman Marc were expecting their first baby togehter, saying: "Time to let everybody know that @cameramarc and I are having a baby in the summer. The three musketeers become four."
The couple got engaged in October 2015 after a year of dating.