The couple, who are already parents to 15-month-old son Noah, have not yet confirmed the baby's arrival, and no other details are known at this time.
Megan, 27, is also a stepmother to Brian's 12-year-old son Kassius with his ex-partner Vanessa Marcil.
In November, she spoke to People about her growing family. "I think Noah is going to be an amazing big brother with time," the star said.
"I think he has an idea, but I don't think he understands that a baby will be living with us all of the time - that will be an adjustment.
"He loves to be the centre of attention," she added. "And I also think he is a really sensitive child. I think that he's going to be a nurturer."
Both Megan and Brian, 40, are notoriously private about their personal life. When she gave birth to her first son in September 2012, they kept the news a secret for two weeks.
"We are humbled to have the opportunity to call ourselves parents of this beautiful soul, and I am forever grateful to God for allowing me to know this kind of boundless, immaculate love," Megan eventually announced in a Facebook post.
Megan and her husband Brian Austin Green
Speaking the following March, the actress admitted, "All I wanted to do my whole, whole life was have a baby and now I've finally done it. I just want to give Noah as much of myself as I can."
Just ten months after the couple welcomed their baby boy, they announced they were expecting again.
"I can confirm Megan is expecting her second child with husband Brian," her rep confirmed. "They are both very happy."
Megan and Brian, who married in 2010, have a busy year ahead. She is starring in the reboot of Teenage Mutant Ninja Turtles as reporter April O'Neill, and later this month will release an Avon fragrance, Instinct For Her.
Brian, meanwhile, will be seen on screen alongside Mena Suvari later this year in the horror film Last Stop. He also has a recurring role on Charlie Sheen's series Anger Management.