The 26-year-old actress gave birth to Noah Shannon Green on September 27.
The couple, who wed in June 2010, kept mum on the newborn for three weeks until finally confirming the news on Megan's official Facebook page this morning.
"We have been very lucky to have had a peaceful few weeks at home, but I would like to release this myself before others do," Megan wrote.
"I gave birth to our son Noah Shannon Green on September 27th. He is healthy, happy, and perfect."
"We are humbled to have the opportunity to call ourselves the parents of this beautiful soul and I am forever grateful to God for allowing me to know this kind of boundless, immaculate love."
"Thanks to those of you who wish to send your positive energy and well wishes. May God bless you and your families abundantly."
It's the first child for Megan, and the second for Brian. The 39-year-old actor has a son, Kassius, from his past relationship with soap star Vanessa Marcil.
Back in April, Megan told Cosmopolitan magazine that she'd like "at least two, probably three" children.
In May, she famously cut an interview short when asked about babies (see below).
We're sure the new addition to the Fox-Green family will positively transform their lives!
What are the best and worst celeb baby names? View our slideshow below, and you be the judge:
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