Toofab reported the news Thursday, adding that no other details about the new baby had been disclosed.
The Transformers actress and the Beverly Hills 90210 alum welcomed their first son, Noah, in September 2012.
In November of last year, Fox spoke to ABC News about her second pregnancy.
"I’ve had rough first trimesters," she said. "But once you get into the second, it’s fine after that."
The new baby is the second child for Fox and the third for Green who also has an 11-year-old boy named Kassius from his relationship to Vanessa Marcil.