The 44-year-old actor discussed his desire for a daughter with Fox in a Facebook Live interview with Hollywood Pipeline on Monday.
"I have four boys now, a fifth is a basketball team," Green told host Dax Holt. "So that would be kind of awesome."
"I want a girl. I really want a girl," he confessed. "I'm worried that if I try I'll just get another boy. Not just, because I love my boys, but I'll have another boy and I don't know if I'm ready for five."
Green shares 4-year-old Noah, 3-year-old Bodhi and 1-year-old Journey with Fox, and 15-year-old son Kassius with ex-partner Vanessa Marcil. He reconciled with Fox last year after temporarily splitting from the actress in 2015.
"Just stick it out. Like fighting and making up, believing in each other," the star advised fans while speaking to reporters in May. "I've dated people before and you kind of get to that point where you go, 'You know what, I don't feel the same way,' and I've never reached that with her."
"I mean we fight, we disagree on things. But I don't want to go anywhere and she doesn't, you know? So we stick it out," he said.
Fox jokingly referenced her family with Green while sharing a throwback photo of the actor Monday on Instagram.
"Thank you for donating your DNA to the shared dream of birthing our own colony of teen heart throbs #mcm #colonymission," the 31-year-old actress captioned the picture.
Green is known for playing David Silver on Beverly Hills, 90210, and Derek Reese on Terminator: The Sarah Connor Chronicles. He has since appeared on Smallville, Desperate Housewives and Anger Management.