"Yesterday Harper got a little brother! We are so excited to introduce Holt Fisher Smith. Our perfect sweet little man. #weareblessed #familyoffour," she wrote next to a photo of her children.
Thiessen, 41, announced in January that she and Smith, 43, were expecting a baby. The "Dinner at Tiffani's" host, who married Smith in 2005, gave birth to Harper in 2010.
"We are excited to announce the news that we are expecting," she wrote on social media at the time. "Harper could not be more thrilled to have a sibling and a playmate."