The Disney star made the announcement Friday on her WhoSay social media account, People.com reported.
In the photo post, she's seen wearing jeans and black sweater looking noticeably pregnant with some of her friends.
"Guess what?! Hint: why am I holding my stomach?" she wrote on the post.
Thornton, 27, and Carney's first child, Kenneth James, is 14 months old, People.com said. The couple married in November 2011.
Swim Week Miami Beach 2014 [PHOTOS]