Mel Gibson is shown in this 2008 file photo. photo by AP
The Australian-raised actor and producer already has seven children - a daughter and six sons - to his estranged wife Robyn.
The couple filed for divorce in April. They formally announced their split earlier this year after 28 years of marriage.
According to celebrity website, RadarOnline.com, Grigorieva gave birth on Friday, a few weeks early. There were no further details.
Grigorieva, a singer signed to 53-year-old Gibson's record label, has a 12-year-old son with former James Bond Timothy Dalton.
