The 'Teenage Mutant Ninja Turtles' star reportedly welcomed Journey River Green on Thursday August 4, with her husband Brian Austin Green, E! News reported.
No other details have yet to be released about the little one.
The gender of the 30-year-old actress' baby was revealed by her father Franklin earlier this year.
He shared: “Megan is an amazing mom and I can't believe she is going to have three boys. Becoming a grandfather has been a wonderful experience and brought us all even closer together.
“All I can say is they are together and they are happy. I've met Brian a tonne of times now and he is a great guy ... I am the grandfather of her two gorgeous boys and now she has another boy on the way. I couldn't be prouder of the woman my little girl has become.”
Meanwhile, it was previously claimed that things are back to normal again for Megan and Brian - who also have sons Noah, three, and Bodhi, two, together - and they are “totally back together” despite Megan filing for divorce last year.
A source said: “They're totally back together. They've been through a lot, but this makes more sense for the kids and the new baby ... Megan is coming around but she hasn’t called off the divorce just yet.
“They recently moved to Malibu together and they've been touring preschools. They're trying to get in as much special time as possible.”
Bang Showbiz