"It’s a girl!" Later wrote under a photo of her super cute baby girl.
"We couldn’t be more excited to share news of the arrival of Vivienne Margaret MacArthur," Larter's caption reads. "She came bearing gifts for her brother: Batman sheets and lots of Gummy Bears!!! 7.5 lbs of love."
Larter, 38, and MacArthur, 37, tied the knot in August 2009, and welcomed their son, Theodore, in December 2010.
Congrats to the happy couple!
Check out the video below for a look at some of last year's biggest and best celebrity pregnancy announcements.