"Welcome Rio! Our first baby girl to join our family!" the 67-year-old actress tweeted at her son.
Hawn's son Oliver Hudson and his wife Erinn Bartlett have two other children. The couple had Wilder Brooks Hudson on August 23, 2007, and Bodhi Hawn Hudson on March 19, 2010. The've been married since June 2006.
Hawn's daughter Kate Hudson, also has two sons, Ryder Russell Robinson, 9, and Bingham Hawn Bellamy, 2.