"Welcome Rio! Our first baby girl to join our family! @theoliverhudson and Erinn thank you for bringing so much JOY to our lives! #happiness," Hawn tweeted Thursday.
Each of Hawn's children -- actor Oliver Hudson and actress Kate Hudson -- have two sons of their own.
Rio is the first daughter of Oliver and his wife Erinn Bartlett, who married in 2006.
NBC reportedly holds celebs hostage to Jimmy Fallon's show
Ray Liotta sues skin care company over use of likeness