Grigorieva gave birth to Lucia in Los Angeles on Friday and mother, father and baby are all doing well.
Gibson's publicist Alan Nierob confirmed Lucia's arrival.
"This will confirm that Oksana Grigorieva and Mel Gibson welcomed the arrival of a baby girl named Lucia on Friday, October 30, 2009, at an undisclosed hospital in Los Angeles," Nierob told People magazine.
Lucia is Gibson's eighth child.
He has six sons and one daughter to wife of 30 years, Robyn, who filed for divorce in April.
Gibson, 53, and Grigorieva, 39, were thrilled with the arrival of Lucia.
"Everything is great. They couldn't be happier," an unnamed friend told People.
"Everyone's healthy and thrilled. He loves it!" Gibson was forced to leave Grigorieva and Lucia in Los Angeles on Sunday and fly to New York to resume shooting Beaver, his new comedy directed by Jodie Foster.
On the set of the movie in White Plains, New York, Monday the crew sang "For He's a Jolly Good Fellow" and Gibson reportedly blushed.
Grigorieva, a Russian-born singer and pianist, has a 12-year-old son to former James Bond actor, Timothy Dalton.
