The couple took to Instagram to announce the newest addition to their family, who is named Eloise Christina Schwarzenegger Pratt.
In separate posts, they wrote of their excitement to welcome their new baby girl, adding: “Mama and baby are doing well. We feel beyond blessed and grateful.”
Pratt, 42, and Schwarzenegger, 32, are already parents to daughter Lyla Maria, who is nearly two years old.
The Guardians of the Galaxy star also shares a nine-year-old son, Jack, with his ex-wife Anna Faris.
Famous friends and fans of the couple showered them with congratulatory messages on Instagram following the announcement.
Under Pratt’s post, Chuck star Zachary Levi wrote: “Congrats brother! Sending love and light to ya’ll!”
Gal Gadot commented: “Mazel tov! Congrats!”, while Despicable Me producer John Cohen left red heart emojis and added: “Congratulations!”
Schwarzenegger’s mother, Maria Shriver, commented on her daughter’s post: “Bravo baby you’re such an awesome mama, your little girl is so lucky.”
Last year, Pratt reflected on what becoming a father of two was like and told E! Newsthat having a second child “really shows you the uniqueness of the first child”.
He continued: “When you have one child, you just think, ‘Oh that’s what babies are like’. Then a second child comes along, and they might do things differently from the get-go, and you say, ‘Oh wow, OK, so that was actually unique what my first child had done’.
“Each child is unique and special and they’re not just punched out of a mould.”
Pratt’s first child with Faris was born prematurely and suffered from a cerebral haemorrhage.
In her podcast, Unqualified, Faris previously opened up about her experience after Jack’s birth and said she was warned that “there was a chance he could be developmentally disabled”.
He required multiple surgeries after his birth and is now healthy as he lives with some vision and leg issues.
Faris and Pratt were married in 2009 and announced they were legally separated in 2017. They were divorced the following year.
Pratt and Schwarzenegger were married on 8 June 2019, while Faris eloped with cinematographer Michael Barrett in 2021.