According to a birth certificate obtained by the site, the Teenage Mutant Ninja Turtles star gave birth to Bodhi Ransom Green on Feb. 12.
Fox, 27, and Green, 40, who wed in June 2010, are already parents to son Noah, 17 months. (Green is also dad to son Kassius, 11, with ex-girlfriend Vanessa Marcil.)
The This Is 40 actress admitted back in November that her second pregnancy was "rough in the beginning." "I've had rough first trimesters," the mom explained to Us of both her pregnancies. "But once you get into the second, it's fine after that."
The couple announced they were expecting their second child together last August. At the time, Fox was busy filming 2014's Teenage Mutant Ninja Turtles in New York City.
Shortly after the announcement, former Beverly Hills, 90210 star Green admitted that they didn't have their hearts set on a boy or a girl. "I'm [just] hoping for a healthy baby," he said. Adding about his pregnant wife: "She's doing great. She did great the first time around, but I wouldn't wish the experience on anybody! I couldn't do it! I couldn't make a baby, it's unbelievable!"
Sign up now for the Us Weekly newsletter to get breaking celebrity news, hot pics and more delivered straight to your inbox!