Deschanel's daughter was reportedly born in Texas where her movie producer husband was based in late July according to the New York Post's Page Six.
A rep for the star also confirmed the happy news to Mirror Celebs, stating: "I am thrilled to confirm that Zooey Deschanel and her husband Jacob Pechenik are new parents to a beautiful healthy baby girl."
The couple announced their engagement in January after it was revealed Deschanel was pregnant with their child.
At the time, Deschanel told PEOPLE: "Jacob and I are over the moon. We are so excited to meet our little one."
And her excitement didn't stop there.
In an interview with E! news in February, the actress said: "I'm really excited. I'm so excited. I'm just so excited to meet my baby! We're very excited."
Deschanel is yet to personally announce the news and reveal the name of her first child.
The newlyweds decided not to find out their baby's gender until the birth. Deschanel revealed during an appearance on Jimmy Kimmel Live! in March that keeping it a guessing game was more "fun".
The baby is the second recent addition to the Deschanel brood as Deschanel's sister, Emily, gave birth to her second son in June.