"We are over the moon!" the couple said in a statement. "It's something we've wanted for a long time and we are thrilled that it's actually happening."
Balancing motherhood with her new role on USA's drama White Collar will be a welcome adjustment for the star, best known for her work on Saved By the Bell and the original Beverly Hills 90210.
"I've never done it – it's going to be a whole new thing," she told PEOPLE in May when talking about the idea of becoming a mom.
Thiessen, 35, and Smith have been married since 2005.