The Kill Bill star welcomed her third child - her first with boyfriend Arpad Busson - on Sunday (July 15), according to Gossip Cop.
The couple, who began dating in 2007, confirmed Thurman's pregnancy back in February.
They were briefly engaged before temporarily ending their relationship in 2009.
Thurman is already a mother to Maya, 13, and 10-year-old Levon, her children with her ex-husband Ethan Hawke.
He went on to marry and have two daughters with their former nanny Ryan Shawhuges, the youngest of which - India - was born in July 2011.