The couple, who have been dating since 2004 and tied the knot in 2010, welcomed son Noah Shannon on September 27, 2012, reported Us magazine.
"I can confirm Megan is expecting her second child with her husband Brian. They are both very happy," the actress' representative said.
Fox is currently in New York City filming 2014's Teenage Mutant Ninja Turtles movie.
The actress previously spoke about her desire to give her son a sibling.
"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. And I want to have more kids. That is where my heart is," she had said.