Max Ellington Nixon-Marinoni was born Monday, UsMagazine.com said Tuesday. No other details about the child's birth were reported.
Nixon, 44, and Marinoni, 43, began dating in 2004 and announced their engagement in 2009.
The actress also has a son and daughter from her previous relationship with photographer Danny Mozes.
Nixon is best known for her work in the "Sex and the City" TV series and films.